Sheldon has a daily 20-minute time period between 2:45 and 3:05, during which he spends locked in a Caltech basement room. Howard and Raj are determined to find out what he is doing. Meanwhile, Leonard is jealous over Penny's (male) college class partner, forcing Penny to accidentally admit something very important.
Sheldon mentions one day in the cafeteria that he could, in theory, build a containment field to open a wormhole to another dimension the size of a Frisbee. When Sheldon leaves, he hands the napkin, on which he has been doodle|doodling some thought on a wormhole generator, to Howard. Howard wants to know if he wants to build it. Sheldon says that he wants him to wipe his chin; he has food on it. After Sheldon leaves, Howard notices that Sheldon's pristine schedule has a lapse of 20 minutes every afternoon where no one knows where he goes or what he does. Raj guesses that he heads to the men's room, while Leonard corrects him since that is not on Sheldon’s pristine schedule. Raj and Howard corner him in the hallway by his office, and Sheldon denies that anything is unusual. They then ask his assistant Alex, and she reveals that even she doesn't know where he goes.
Later, Penny and Leonard are hanging out in her apartment. Penny is texting a fellow English student named Cole, who's working with her on an oral class project. Jealous, Leonard asks if he and Penny are still dating and is convinced that Cole is flirting with Penny, but Penny denies it and says that he's just being nice.
Leonard begins chatting with Alex in the Cafeteria about it, and she mentions that it is probably nothing. She then coyly adds that Leonard might even be flirted with without really knowing it. She almost admits that she is purposely flirting with him. He the walks out with his coffee laughing hysterically while Alex looks disappointed.
Elsewhere at Caltech, Raj and Howard follow Sheldon down to an old storage room in the basement. Unable to hear what he's doing, Howard and Raj break in later that night and find an almost empty room with a blackboard that has the number 43 written on it. Given Sheldon's apparent obsession with order and numbers, Raj and Howard spend all night trying to figure out what the significance of the number 43 is. Howard's mom has 43 inches of neck fat.
Later, when Cole comes over to Penny's to work on the project, Leonard spies on him from his apartment. Leonard follows him down the stairs and tells him to watch out for "Penny's boyfriend", saying that he was a gang member. When Cole says that Penny told him that her boyfriend was a scientist, Leonard makes the excuse that "The Scientists" was the name of his gang. Penny
witnessed his stunt.
Penny is very upset with Leonard, and the two later meet in the hallway and have an argument. Penny tells him that she cannot believe that he cannot trust her and that he embarrassed her in front of Cole. who knew who he was since his picture is on her refrigerator. Penny continues to tell him that he is the one she is with and accidentally mentions that he knows that she loves him. Leonard is stunned and tells her that that is the first she ever said that. Penny rushes off to work to avoid crying in front of him, also allowing Leonard to run off and cry, too. Leonard walks into his apartment sniffling and says to himself that she loves me. Next he gets a text from Alex, inviting him for coffee sometime, and Leonard smiles, mentioning that she is such a friendly girl.
Unable to solve what 43 means, Howard installs a spare camera from the Mars rover project so they can spy on Sheldon in the storage room. The next day, Howard and Raj watch the video feed and are stunned when they see Sheldon use an unknown machine to create a small wormhole in the room. Sheldon mentions aloud into a recorder that he hasn't found life in 43 parallel universes, and thrusts his head through the wormhole. Sheldon backs out, screaming as a reptilian creature eats his face. Raj and Howard scream as Sheldon's prop for the creature is dropped on their laptop from behind them and they scream again. Sheldon had found the camera and created a phoney video to cover up what he was really doing. Sheldon explains that what he did with his spare time was none of their business, but was centred around giving his mind a break from his struggles with understanding social behaviour and the mental bombardment caused by his work. Later, Sheldon is seen in the storeroom playing with a Hacky Sack, telling himself that he'll never reach 43 consecutive hits again.
- Guest starring:
- Teleplay: Steven Molaro, Jim Reynolds & Steve Holland
- Story: Chuck Lorre, Dave Goetsch & Anthony Del Broccolo
- The TV Critic: "The Leonard and Penny story had a very pleasant conclusion which rather justified yet another story about Leonard’s inadequacy. It wasn't just that she finally said I love you. It was also that he was able to admit the source of his jealousy (guys constantly hitting on her) in a way which made sense and then did the mature thing of admitting that it was all in his head and that it wasn't her fault...Sheldon having a secret room was always going to be a fun bit of intrigue. The success of that story was always going to be about how much you enjoy Howard and Raj bantering...This was pleasant but no more."
- The A.V. Club gave this episode a D+.
- IMDb user reviews
- Title Reference: The title refers to the number written on Sheldon's blackboard in his secret basement hiding spot.
- Chuck Lorre's vanity card.
- This episode was watched by 17.63 million people with a rating of 5.7 (adults 18-49).
- Total viewers including DVR users 21.54 million.
- This episode aired in Canada on November 15, 2012 with 3.86 million viewers and a weekly ranking of #1.
- In the United Kingdom, this episode aired on January 10, 2013 with 2.754 million total viewers.
- Episode transcript 
- Sheldon wears his orange Space Invaders shirt and one of his DNA shirts.
- Raj's murder mystery parties are mentioned in this episode, which the gang is forced to participate in during "The Mommy Observation" (S7E18)
- Leonard is growing isotopically pure crystals for neutrino detector|neutrino detection.
- Howard states, "Ah, it took him two years to put together that LEGO Death Star, I’m not worried." The Lego Star Wars|LEGO Star Wars Death Star was previously seen in "The Isolation Permutation". LEGO Star Wars was also referenced in "The Weekend Vortex".
- First time Melissa Rauch has been absent from an episode since "The Rhinitis Revelation" (S5E6). There is also no mention of her character.
- First time Mayim Bialik has been absent from an episode since the beginning of the sixth season. There is also no mention of her character. The only other such episode is "The Proton Resurgence" (S6E22).
- First episode since season "The Prestidigitation Approximation" (S4E18) to feature just the five original cast members.
- Raj questions whether Sheldon chained up Leonard Nimoy, Bill Gates, or Stephen Hawking in an old storage room at Caltech. Howard then asks, "Why would he chain up Stephen Hawking?," with Raj responding, "Howard, please, you can’t treat the man differently just because he’s disabled. That’s not okay." Leonard Nimoy was also mentioned in "The Bath Item Gift Hypothesis", "The Excelsior Acquisition", "The Recombination Hypothesis", and "The Hawking Excitation", while he provided the voice of Spock in "The Transporter Malfunction". Bill Gates was likewise referred to in "The Large Hadron Collision" and "The Toast Derivation". Stephen Hawking was also mentioned in the "Pilot", "The Bath Item Gift Hypothesis", and "The Agreement Dissection", seen in "The Hawking Excitation", and heard in "The Extract Obliteration".
- It's unclear why Raj would mention Sheldon having Leonard Nimoy chained up, especially considering that Nimoy placed a restraining order on him.
- Penny "accidentally" tells Leonard that she loves him while she is arguing with him which shocks both of them. This time is the first time she says "I love you". Leonard tells her he loves her the first time in "The Wheaton Recurrence" which was three seasons ago.
- This episode is the most-watched episode of the series to date, with an unprecedented 17.4 million people tuning in, for a 5.5 rating, in the United States, and the Canadian simulcast had 3.86 million, for a total audience of 21.26 million measured, sanctioned viewership of the November 15, 2012 world telecast premiere of this episode.
- The cast and crew got together and performed a flash mob dance for the audience during the live taping of this episode. It can be seen on YouTube.
- Kaley Cuoco admitted that during the scene where Penny told Leonard that she loved him, that that was the time she really felt like she was Penny and that it was like a real life moment in Leonard and Penny's world. Both Johnny Galecki and Kaley felt that they were alone doing the scene without noticing the production staff and the audience and did it in one take without ever looking back.
- Howard calls Raj 'dumbass', which was Leslie Winkle's nickname for Sheldon earlier in the series.
- Though this episode uses the number "43" as a plot device, "73" is Sheldon's favorite number per "The Alien Parasite Hypothesis" and has it emblazon on a T-shirt.
- In a number-letter code (where 1=A,2=B and so on), 43 can actually be translated to d(4)c(3). Since Sheldon likes DC Comics, this could be a coincidence or an intentional Easter Egg thrown in by the producers.
- Leonard: Hi.
- Penny: Hey. Shouldn't you be out with your gang spray-painting equations on the side of buildings?
- Leonard: Come on, I’m sorry.
- Penny: I just can’t believe you don’t trust me.
- Leonard: I feel... Of course I do.
- Penny: Then why did you embarrass me in front of my friend who, by the way, knew exactly who you were?!
- Leonard: Really?
- Penny: Your picture’s on my refrigerator!
- Leonard: Oh. You know, I’m really starting to not like this guy.
- Penny: What is your problem? Do you use up all your thinking at work and then have none left over for when you get home?
- Leonard: I don’t know. It’s hard sometimes. Everywhere you go guys hit on you, even if I’m standing right there. And they are all taller than me. Why is everyone always taller than me?! You know what? This is all in my head. It’s my problem, not yours.
- Penny: Leonard. Why do you always do this? Listen to me. You are the one I’m with. You know I love you. So would you please relax--because you are driving me crazy!
- Leonard: You know that’s the first time you ever said that you love me.
- Penny: Yeah...
- Leonard: We’re supposed to pretend it’s not a big deal?
- Penny: That’s exactly what we’re going to do... because you’re about to make me cry and we both know that if I start crying, you’re going to start crying.
- Leonard: You’re right, you should go.
- Penny: Fine! [Runs away downstairs]
- Leonard: [Enters apartment] She loves me. (gets a text and reads it) "Hey, it's Alex. Nice having coffee with you. If you wanna talk more, I'm always available. Smiley face. Smiley face." (smiles) What a friendly girl.
- Alex: Oh hi, Dr. Hofstadter.
- Leonard: Oh, hey, Alex. And call me Leonard. Dr. Hofstadter is my father. And my mother. And my sister. And our cat. Although I’m pretty sure that Dr. “Boots” Hofstadter’s degree is honorary.
- Alex: May I join you, Leonard?
- Leonard: Sure. Um, let me ask you something. My girlfriend knows this guy at school. He’s got an English accent.
- Alex: Ooh, I love English accents.
- Leonard: Yeah, you all do. Anyway, I feel like he’s hitting on her. She says he’s just being nice and I should trust her.
- Alex: It’s probably harmless. You know how it is. I’m sure you get hit on all the time.
- Leonard: Right. Because girls are always like: Ooo, that guy owns two Star Trek uniforms and gets lots of ear infections. "I got to get me some of that."
- Alex: You know, I bet it happens more than you realize.
- Leonard: Trust me. It doesn't.
- Alex: You sure? You’re cute. You’re funny. Maybe you’re getting hit on and you don’t even know it.
- Leonard: Really?
- Alex: Yep, pretty sure.
- Leonard: (Laughs) I have to get back to work. Thanks for listening.
- Alex: No problem.
- Leonard: Hope no girls rip my clothes off along the way.
- Alex: All I know is corduroy makes too much noise and I have to go find quieter pants.
- Sheldon (On screen): This is Dr. Sheldon Cooper. Experimental log. Wormhole generator test forty-four.
- Howard: Wormhole generator test? (On screen a wormhole appears)
- Sheldon: The first forty-three parallel universes I've checked proved to be empty. I see no reason to suspect universe number forty-four will be any different. (Puts his head in the wormhole)
- Howard: Oh, my God!
- Raj: Holy crap!
- Sheldon (With an alien creature stuck to his face): Oh! It’s eating my face!
Taping report by megfsc
- The Big Bang Theory - The 43 Peculiarity (TV Episode 2012) - IMDb
- The TV Critic's Review
- The Big Bang Theory: “The 43 Peculiarity” - TV Club - The A.V. Club
- Live+7 DVR Ratings: 'Modern Family' Leads Adults 18–49 Ratings and Tops Viewership Gains; 'Private Practice' Earns Biggest Percentage Gains in Week 8 - Zap2it
- Top Programs – Total Canada (English) November 12 – November 18, 2012 - BBM Canada
- BARB via Wikipedia