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"The Recombination Hypothesis" is the thirteenth episode of the fifth season of the American sitcom The Big Bang Theory. This was the show's 100th episode, and first aired on Thursday, January 19, 2012.[1]


Leonard asks Penny out on a spur-of-the-moment romantic dinner for two, as they contemplate renewing their relationship in spite of the discouragement of his friends.

Extended Plot[]

BBT - Sheldon and lifesized cutout

Sheldon inspects his Spock cutout.

Sheldon and Leonard are walking up the stairs to their apartment. Leonard asked Sheldon whether he will consider changing their dinner menu, for once, to Indian food or Tex Mex. Sheldon ignores the question and instead asks Leonard what he thinks if humans evolved from lizards instead of mammals. Leonard sighs and lets Sheldon continue with his rant as Sheldon gives factoids about lizards and how humans would be should they have evolved from lizards and ends up impressing himself and saying he loves his mind. Leonard responds sarcastically by saying that they all love his mind too and brings back the conversation topic to dinner. Sheldon is still into lizards as they reach their apartment. There is a package by the door and Sheldon gets excited to find that it's his new life-size Spock stand-up display. He starts removing it from the box while, across the hall, Penny's door is open and Leonard stares at her as she takes out the contents from a box with Amy and Bernadette standing next to her. The two smile at each other. Sheldon is still into his Spock stand-up so Leonard excuses himself and walks over to Penny's door. The girls are listening to Amy's monkey poop-flinging story, but are interrupted when Leonard stops by the door and asks Penny if she has plans for dinner. Penny asks if the guys are going somewhere, but Leonard clarifies that he is referring to the two of them only. Penny is shocked and asks if Leonard means it is like a date, but Leonard immediately emphasizes that it is a date.


Amy and Bernadette get excited (OOOOOO!!!)

Amy and Bernadette get excited (OOOOOOH!!!) and Penny agrees. Sheldon suddenly complains that he has been delivered the wrong Spock since he ordered the Leonard Nimoy version, but received the Zachary Quinto version from the recent film instead

Amy and Bernadette sit on Penny's bed as she tries out some fancy clothes for her date. Bernadette tells Amy that it is so exciting since, if Penny and Leonard get back together, they can double date with her and Howard. Amy asks how come she never asked her and Sheldon for a double date. Bernadette looks uncomfortable and quickly changes the topic by asking Penny how she is doing with the clothes. Penny comes out of the bathroom in a sexy green dress and asks the girls if they think it is too much.

Too much

Too much?

Bernadette says yes at the same time Amy says no. Penny goes back in to change. Amy says Bernadette just can't handle Penny's raw sexuality. Bernadette asks Penny when she and Leonard broke up. Penny replies about two years ago and Bernadette wonders why Leonard suddenly asked her out again. Penny does not know. Amy suggests maybe Leonard is dying and that it would be so romantic. Penny dismisses the idea and comes out wearing something loose and unflattering and asks the girls their thoughts about it. Bernadette says maybe at the same time Amy says "come on".

How about this

How about this?

Penny goes back in to change again. Bernadette asks whether Penny will sleep with Leonard tonight, but Penny says no since they are just having dinner and will see how it goes. Amy asks if Leonard was dying would she sleep with him to which Penny repeats that Leonard is not dying. Amy asks her how she would know since she is not a doctor. Bernadette suggests they can do it in a make-believe style, telling about her own make-believe sex with Howard.

Naughty part

And the naughty part is I'm not in his HMO network.

Penny comes out in her initial blue-green t-shirt and jeans and tells them she is not making a big deal out of this date since it is just dinner.

BBT - Penny is nervous

Penny is nervous about the date.

Amy adds "with a dead man" and Penny tells her to stop it and that she is nervous. Bernadette tells her to relax since Leonard has always been crazy about her so the date will go well. Penny tells them that, after so long since their break-up, they can finally hang out together without it being weird and fears what will happen should the date go bad. Amy tells her then it would just be her and Sheldon to double-date with Bernadette and Howard. At the thought of that, Bernadette stands up and tells Penny to change her clothes because there is a lot riding on tonight.

The Recombination Hypothesis Sheldon Howard and Raj

Sheldon being teased over the pun "wood" over the game The Settlers of Catan.

At the guys' apartment, Sheldon, Howard, and Raj are playing The Settlers of Catan. Sheldon says he needs wood and Howard and Raj laugh. Sheldon does not understand the laughter and tells them that the objective of the game is to build roads and settlements and for that he needs wood. He already has sheep so he needs wood for his sheep. The guys burst out laughing at the sexual innuendo of "wood". Leonard comes out from his bedroom all dressed up for his date. Howard asks him why he is going out, but Leonard doesn't understand. Raj reminds him that, when he broke up with Penny the last time, it took him two years and defiling Raj's sister Priya for him to get over it. Leonard says he did not defile Priya since they had a relationship. Raj tells him he heard him call her "Brown Sugar" and that is defilement in his book. Sheldon interrupts with his own opinion on the matter saying that the reason Leonard is attracted to a good-natured simpleton like Penny is because she is the opposite of his first romantic attachment, his mother. Leonard asks where Sheldon got that and Sheldon says from his mother's latest book Needy Baby, Greedy Baby. Leonard says that doesn't make it true and Sheldon tells him the book is non-fiction for a reason. Leonard just says goodbye and leaves. Raj comments that if there is a movie version of the book, Sandra Bullock should play Leonard's mother. Howard asks why and Raj says Sandra's great in everything. Sheldon shakes his head and brings the topic back to their card game saying he wants wood. The guys start laughing again. Sheldon tells them that he just wants wood so why are they making it so hard, which makes them laugh even harder.

The Recombination Hypothesis Penny and Leonard on a date

Leonard and Penny on their date.

On Leonard and Penny's date, they are looking over the menus. He asks her whether there is a name for a first date with someone you used to date. Penny says "how about awkward". Leonard agrees and suggests they pretend as if this night is their first date and Penny agrees. Leonard pretends he does not know Penny, calls her "Polly", and asks her to tell about herself. Penny corrects her name and tells about her coming from Nebraska, her dream to become a movie star, and her current job as a waitress. She asks Leonard about himself and he tells her he is an experimental physicist at Caltech and his work involves the usage of high-powered lasers and that recently he just received a government grant to find out whether the lasers can be used to knock out incoming ballistic missiles. Penny asks whether it is possible and Leonard says no, but he uses the equipment to make his own bat signal. Penny laughs and asks whether he is some kind of nerd. Leonard replies that he is not just some kind of nerd but that he is the "King of Nerds". Penny laughs and asks what does that mean. Leonard says that if anyone displeases him, he will not help them set up their printer. Penny says Leonard is funny and she is glad they are going out. Leonard then asks whether they are getting back together, but Penny is taken aback and tells him not too fast and that she will always have feelings for him. Leonard gets upset and says the word "always" is used to cover up the real meaning behind the fact that one does not want a relationship with someone. Penny asks him how he would say it so Leonard uses the words "have feelings" instead. Penny says it is the same thing and that Leonard is over thinking things. They start arguing and Penny points out they always have problems when they talk. Leonard asks how are they supposed to be in a relationship if they do not talk and Penny tells him she once dated a guy named T.J. for 8 months and they never talked. To this day, she still does not know what T.J. stands for. Leonard starts to ask what they did if they didn't talk, then stops himself and calls it a stupid question.

At the guys' apartment, over the card game, Sheldon says that now that he has wood, he can start the erection of his settlement. Raj tells Howard that Sheldon must be doing it on purpose. Leonard comes in, and Howard starts teasing him about ending the date at only 8:30 p.m. He must have painted the town beige. Leonard retaliates that Howard is 30 years old and still lives with his mother and walks to his room. Raj suggests the date must have gone bad. Sheldon says they do not know that and the only thing certain is that Leonard is home. Leonard starts screaming from his room so Howard asks Sheldon what he thinks about that. Sheldon says there is still insufficient evidence and, for all they know, Leonard is being murdered. Sheldon tells them to get back to the game and Raj playfully reminds him that he is in the middle of an erection. Sheldon says of course and that it is right there in his hand. The guys laugh again.

What does she want now

What does she want from me now?.

Later that night, Leonard is sleeping in his bedroom and gets a text from Penny asking to meet him in the hallway right now. Leonard walks out to meet Penny who was waiting for him in front of his apartment. He asks her what is the matter and she tells him not to over think this, and immediately kisses him.

Do not overthink this

Do not overthink this.

She then pulls his hand leading him towards her apartment, when Leonard starts talking and she tells him to be quiet, no talking. Later, they are lying on her bed together.

BBT - Leonard and Penny in bed

Well, that was fun.

Penny tells him it was fun and he agrees, except when he got the foot cramp. Penny complements him on his ability to "hang in there," to which Leonard replies that there was a lot riding on it. She asks him what he wants to do now and he tells her he wants to get his asthma inhaler, but it might ruin the moment. He asks her how did a bad date end up with them in bed together. Penny says it is complicated, but he says he's a smart guy, and right now his brain has dibs on the blood supply, so give it a go. Penny says she was scared back at the restaurant when she thought things were going too fast. He asks her what she was afraid of. She tells him what if they go out again and she ends up doing something stupid like dumping him again. Leonard says he could be the one doing the dumping this time. Penny tells him to be serious.


Penny remembers she still has one of his inhalers.

He suggests they not look at it as a relationship but like a new version of software instead, a "Penny and Leonard 2.0". They test it out, shake out the bugs, and when they think it is solid enough, go public with it. She asks him if he wants them to keep it from their friends. He says yes and tells her to let them know that the date went bad but they decided to remain friends. Penny agrees and says she remembered something, pulling out one of Leonard's inhalers from her bedside drawer. Leonard cannot believe she still keeps it. She tells him she tried to throw it away but couldn't do it. Leonard thinks it's sweet and uses the inhaler. He then tells her that he is going to show her how much he appreciates this in about 30 minutes.

Leonard sneeking home

Sheldon catches Leonard sneaking back into the apartment.

Later, Leonard sneaks back into his apartment, only to encounter Sheldon, who comes out from the bathroom. Sheldon asks him what he is doing out at 3 a.m.. Leonard twists the question and asks him what he was doing. Sheldon says he was using the bathroom and Leonard said he was too. Sheldon says he did not see Leonard in there so Leonard answers by saying, since the bathroom was occupied; he used a different one at the gas station across the street. Sheldon asks whether he did so in his pajamas, with no shoes, and on a cold winter's night. Leonard says yes, but Sheldon does not seem to buy it and asks whether he brought along his inhaler. Leonard takes out his inhaler from his pocket that Penny gave him and shows Sheldon. Sheldon says then it is plausible, but then asks Leonard again if he bought something. He explains that, it is customary if one uses the facility at a retail establishment, one should make a small purchase. Leonard said he did not so Sheldon gives him $2 and tells him to go back and buy some beef jerky. Leonard says he doesn't want beef jerky, but Sheldon tells him that this matter is not about him, but about a poor immigrant from Pakistan] trying to make his way in America by working the graveyard shift. Leonard says fine, takes the money, and walks out. Sheldon says Leonard is indeed "needy baby, greedy baby".

BBT - At the Cheesecake Factory

Leonard starts a fight with Penny in front of their friends to hide their relationship.

The gang is at The Cheesecake Factory. Sheldon comes and sits down at their table, telling them Leonard is looking for a different parking space. Howard asks why and Sheldon says the original space has a car next to it with a bumper sticker that says "Ask me about my grandchildren" and he did not want to do that should he run into the car's owner. Bernadette says it is disappointing that things did not work out between Leonard and Penny. Amy agrees and tells Sheldon that, because of this fact, the two of them should not flaunt their happy relationship in front of them. Sheldon nods and notes that Amy has a keen insight into the human heart. Leonard comes in and sits down. Howard says he is surprised Leonard does not mind coming here after how bad his date with Penny went.

18Weponized smallpox

I'm a very vengeful person; with access to weaponized smallpox.

Leonard says that they are grown-ups and can still be friends. Bernadette tells the group that she could not do the same thing with Howard if they broke up. Howard asks why and she says she is a very vengeful person with access to weaponized smallpox. Penny comes and starts pouring them water to get them started on dinner, asking them whether they know what they want to order. Howard tells her that even if Leonard wants something he could not order from her. Penny says it's fine since things do not always work out. Leonard agrees and says they will always have feelings for each other. Bernadette looks uncomfortable, but comments that is nice. Leonard tells Penny that Bernadette understands the meaning of "always". Penny says Leonard does not know how to let something go. Sheldon interrupts and tells them before they both say something they will regret; he wants to place his order. Penny tells him to hang on and proceeds with her argument with Leonard, pointing out how Leonard needs to be right all the time. Leonard says that is not true and Sheldon agrees by saying Leonard is wrong more than anyone he knows. Penny says she just remembers she is on a break and will get someone else to wait on them, and walks away. Sheldon tells Leonard that, when the new person comes, he needs to promise not to start a complicated on-again off-again relationship with the person since he is really hungry.

Later, back at his apartment, Leonard is sleeping when he again receives a text from Penny asking to meet him in front of his apartment. He goes and Penny demands to know why Leonard was a jerk at the restaurant and he says he was pretending they were not seeing each other, since that was the plan. Penny says no, and that the plan was to tell people they are still friends, but that is hard now since he was being such a "tool-bag". Leonard yells that he does not need to stand there and take this treatment and walks towards her apartment. She asks what he's doing so he says that isn't their relationship about having sex after fighting. She says "yes" and follows him into her apartment. Later, as they are kissing on her bed, he asks her what they are doing, since every scenario that he plays out about them ends badly. She says it is because he over-thinks things. Suddenly, Sheldon's voice calls Leonard's name...

Penny happy

Penny, happy Leonard asked her out again.

Leonard snaps back to reality. His entire interaction with Penny from the moment he asked her out was a daydream. He is still standing in the hallway, looking at Penny's apartment, while Sheldon has just unwrapped his Spock stand-up. Sheldon complains that they sent him the wrong one since he wanted the Leonard Nimoy Spock, not Zachary Quinto's, and how would he feel safe with Zachary Quinto at the foot of his bed. Leonard convinces him to give it a try and excuses himself and walks to Penny's door. He really asks her whether she has plans for the night and she asks whether the guys are going somewhere. He tells her he means just the two of them and Penny asks whether he has thought things through. Leonard says yes and he thinks they should go ahead anyway. Outside in the hall, Sheldon says he will just send back the Zachary Quinto Spock stand-up. Penny smiles at Leonard happy that he asked her out.

Penny daydream

Penny has her own daydream.

In Penny's bedroom, as she is choosing an outfit to wear to the date, Amy tells her that if she gets back together with Leonard, they can go on a double-date with her and Sheldon. Bernadette asks what about her and Howard, and Amy says they can come too, but looks at Penny and tells her Bernadette is feeling insecure. Bernadette asks Penny where their date is going to be, but Penny is daydreaming on her own. In it, she sees herself in a wedding dress and heavily pregnant with Leonard at the altar.

Little late

Penny's fear of being pregnant when she gets married.

The minister asks her if she would take Leonard to be her lawfully-wedded husband. Penny rubs her large pregnant belly and tells him that it is too late to start saying "no" now. Penny snaps out of her daydream by Amy calling her name. She apologizes and tells the girls she just remembered that she needs to go to the drug store presumably for contraceptives.



  • The TV Critic: "TBBT rarely leaves its standard perspective or presentation so almost any change has me intrigued... [The] twist was well executed. The reality we saw felt very plausible. Initially Leonard seemed more confident than usual. But as soon as things started to go well he pushed for more and scared Penny off... This really was a nice departure from the norm. If next week we get proper follow up and a detailed discussion of Leonard and Penny's relationship then that would be great. But after five seasons of "character development" we still don't know much about Penny. She's able to sum herself up with one play and one commercial and that's it."[2]
  • The A.V. Club gave this episode a C[3]
  • IMDb user reviews


  • Title Reference: The title refers to the hypothesis Leonard has that he and Penny should go on a date, or perhaps even recombine as a couple again.
  • Chuck Lorre's vanity card.
  • This episode was watched by 15.83 million people with a rating of 5.3 (adults 18-49).[4]
  • This episode aired in Canada on January 19, 2012 with 3.534 million viewers and a weekly ranking of #1.[5]
  • In the United Kingdom, this episode aired on April 5, 2012 with 2.107 million total viewers and a weekly ranking of #2.[6]
  • In Australia, it aired on February 13, 2012 with 1.491 million viewers.[7]
  • Episode transcript [1]

Costume Notes[]

  • To mark 100 episodes, Sheldon, Penny, and Leonard, wore the first outfit they ever wore as seen in the Pilot.
    • Sheldon wears his Flash distressed athletic jersey tee (which is a resemblance of the first outfit) and his orange Aquaman II t-shirt.
    • Penny wears her exact light-blue shirt with darkish-red leaves pattern.
    • Leonard wears a grey jacket with a green hood, and a red t-shirt which may or may not be a t-shirt featuring the capsaicin molecule (the stuff that makes chili peppers hot)


  • This marks the 100th episode of the show, which began on September 24, 2007.
  • The opening scene in this episode is similar to the pilot episode, with Leonard and Sheldon observing Penny through her open apartment door, followed by Leonard going to talk to her. In addition, the characters are wearing similar items of clothing; Penny, in fact, is wearing the exact same top in this scene that she wore in the pilot. In a later scene, Howard and Raj are also dressed in clothes similar to the ones they wore in the pilot.
  • Amy talking about feces-throwing monkeys first appears in "The Zarnecki Incursion" (S4E19).
  • Sheldon assumes that Leonard's first romantic attachment was his own mother.
  • Sheldon is disappointed to discover that a 73"x19" Mr. Spock cardboard stand-up he ordered is a portrayal of Zachary Quinto's Spock from the J.J. Abrams 2009 Star Trek film, not of Leonard Nimoy's Spock from the original series. According to Bill Prady, approval from Paramount, J.J. Abrams, and Zachary Quinto was required in order to mock Spock.
  • Beverly Hofstadter just released a new book entitled Needy Baby, Greedy Baby.
  • Raj is a big fan of Sandra Bullock.
  • During Leonard's daydream, Penny refers to Leonard overthinking things 3 times.
  • This is the first episode in the entire series that used a dream sequence-style of storytelling. The only episode which shared a similar sort of style was "The Staircase Implementation" (S3E23), where most of the story consisted of flashbacks.
  • Penny talks about appearing in an Anne Frank play above a bowling alley this fact may be a reference to the television series 8 Simple Rules where Kaley Cuoco appeared in an Anne Frank play. It also references to the play she did, mentioned in "The Terminator Decoupling" (S2E17) and "The Psychic Vortex" (S3E12).
  • During Leonard's daydream, he does realize that he over thinks their relationship. This action is characteristic of Leonard, especially in the episodes "The Fuzzy Boots Corollary" (S1E3), "The Hamburger Postulate" (S1E5), "The Tangerine Factor" (S1E17), "The Monopolar Expedition" (S2E23), and "The Ornithophobia Diffusion" (S5E9).
  • Amy says "so insecure" to Bernadette similar to how she said "so needy" to Penny in "The Toast Derivation" (S4E17).
  • In Penny's list of her acting accomplishments, she failed to mention her role in the musical Rent that Sheldon and Leonard so desperately tried to avoid due to her singing in "The Loobenfeld Decay" (S1E10).
  • In the dream sequence, while Penny is trying on clothes, Bernadette mentions that if Penny and Leonard get back together that she and Howard will have someone to double-date with. At the end in real life, while Penny is looking at clothes, Amy mentions the same thing with Bernadette replying, thus reversing the exchange in the sequence that takes place in reality and not in Leonard's head.
    • It would make sense for Amy to be the one who'd want to double date with Penny and Leonard, as Bernadette and Howard have already been on double dates with the reunited couple (even their first date was a double date with them).
  • Penny shows her fear of getting married because she is pregnant like other members of her family. She mentions this more than once after she gets engaged to Leonard in season 7.
  • Penny and Leonard get back together after breaking up in "The Wheaton Recurrence" (S3E19).


Amy: Which is why the more intelligent the monkey, the more faeces they fling.
Leonard: Penny, do you have plans for dinner tonight?
Penny: You guys going somewhere?
Leonard: No, I mean, just you and me.
Penny: Like a date?
Leonard: Not like a date, a date.
Amy and Bernadette: Woooooooooo!!!

Penny: I'm not gonna make a big deal out of this. It's just dinner!
Amy: With a dead man. Roar!

Sheldon: The reason you're fixated on a good-natured simpleton like Penny: She's the exact opposite of your first romantic attachment--your mother.

Leonard: So what do you think? Are we gonna get back together?
Penny: Woah!

Howard: Ooh, 8:30. You and Penny decide to go out of town and paint the town beige?
Leonard: You're 30 years old and you live with your mother!
Raj: Guess it didn't go well.
Sheldon: Ah, we don't know that! All we know is that Leonard is home.
Leonard: (from his room) AHHHHHHHHHH!!!
Howard: How about now?
Sheldon: Again, not enough evidence. For all we know he's being murdered.

Sheldon: (excited about his life-sized cutout of Spock) Oh goody, oh goody, oh goody!

Penny: Do not over think this! (puts her arms around Leonard and plants a huge kiss on him)

Leonard: So, uh, do they have a name for a first date with someone you used to go out with?
Penny: Oh, that's a good question. How about awkward?
Leonard: That sounds right. (pauses) Hey, how about if we pretend we're actually on a first date? See how that goes.
Penny: Okay.
Leonard: So, Polly, tell me about yourself.
Penny: It's Penny.
Leonard: Oh sorry, yeah, awkward...
Penny: (laughs) Okay let's see, I'm from Nebraska, and ever since I was little girl I dreamed of moving to LA and becoming a movie star. Anyway, after four years of acting lessons and waiting tables, I've done a hemorrhoid commercial and a production of Anne Frank above the bowling alley. So, you know, dreams do come true! Your turn!
Leonard: Ah, let's see, I am an experimental physicist at Caltech. Most of my research is with high-powered lasers and, oh, I've just gotten a big government grant to see if they can be used to knock out incoming ballistic missiles.
Penny: Wow! Can they?
Leonard: Oh, God no! (Penny laughs) But the money's pretty good, and I used the equipment to make my own bat signal.
Penny: (laughs) Bat signal? What are you, some kind of nerd?
Leonard: Not some kind of nerd, I am the king of nerds!
Penny: (laughs) What does that mean?
Leonard: Uh, it means, if anyone displeases me, I don't help them set up their printer.
Penny: (laughs) You are so funny!
Leonard: Good! Remember that when I take my shirt off.

Sheldon: Do you ever wonder how humans would be different if they evolved from lizards instead of mammals?
Leonard: Okay, let's talk about that.
Sheldon: As you know, lizards, cold-blooded animals, lack the ability to sense temperature. But they do move more sluggishly when it's cold. So, lizard-weatherman would say thing like, Bring a sweater. It's slow outside. I love my mind.
Leonard: We all do.

Sheldon: Oh no! They sent the wrong Spock. Live long and suck it, Zachary Quinto.

Amy: You just can't handle her raw sexuality, can you?

Bernadette: Why do you think he asked you out again?
Penny: I don't know!
Amy: Maybe he's dying! That would be so romantic.

Sheldon: I want to build a road, but I need wood. Do either of you fellas have wood?
(Raj and Howard struggle to contain their laughter)
Sheldon: I don't understand the laughter. The object of Settlers of Catan is to build roads and settlements. To do so requires wood. Now, I have sheep but I need wood. Who has wood for my sheep?
(Raj and Howard burst out laughing)

Leonard: I didn't defile your sister. We were in a relationship.
Raj: I heard you call her Brown Sugar. That's defilement in my book.

Raj: If they ever make a movie version of that book, do you know who should play Leonard's mother? Sandra Bullock.
Howard: Why?
Raj: Because she's great in everything.

Minister: Do you, Penny, take Leonard to be your lawful wedded husband?
Penny: (turning, heavily pregnant): Well, it’s a little late for me to start saying no, isn't it?

Leonard: You know what? I don't have to stand here and take this crap.
Penny: Where the hell do you think you're going?
Leonard: Isn't sex after fighting kind of what we do now?
Penny: Well, kind o

f, yeah.



TBBT 5X13 The Recombination Hypothesis Promo (CTV) - Canadian Version


The Big Bang Theory 5x13 Promo The Recombination Hypothesis