"The Propagation Proposition" is the twelfth episode of the twelfth season of the American sitcom The Big Bang Theory. The episode aired on Thursday, January 10, 2019.


Penny's ex-boyfriend, Zack, and his wife want a baby, but when Zack is infertile, they ask Leonard for help. Also, Raj attempts to mend things with his ex-fiancée, Anu.[1]

Extended Plot

The Posse is at an old bar haunt celebrating being together. The guys are back in Apartment 4A playing Dungeons and Dragons. Raj is getting kidded about spying on his ex-fiancée. The girls are sent a free bottle of champagne from a stranger who turns out to be Zack Johnson. He had sold his company, bought a boat and gotten married. Penny gets an invitation for her and Leonard to have dinner on their boat.

The return of Zack: married and rich.

Penny returns home and feels that she shouldn't have drove since she was laughing at her husband's D&D jokes. Amy had gotten drunk and kept talking about how smooth Sheldon’s chest was. Leonard is a bit jealous over Zack retiring wealthy though he thinks that Zack should be the jealous one since HE got Penny.

Anu hears her doorbell ring and sees Raj out front with poster signs ala “Love Actually”. She tells him to go away and has nothing to say to him. Raj apologizes; however, Anu doesn't think she has anything to apologize to him about. She feels that he wants to get married because all his friends are married. Then Anu agrees that she was doing the same thing wanting to get married now because all her past relationships failed. They seem to part amicably agreeing that they shouldn't get married.

Driving to work Leonard asks Sheldon if he remembers Zack. Sheldon claims that he can list all of Penny's boyfriends in alphabetical order including two named Zeke. Leonard admits that he might be jealous of Zack's success since he is smart and works hard. Sheldon isn't impressed with Leonard's assessment or that Leonard wants to appear mature and confident in front of Penny.

The Johnsons want to ask Leonard something.

Raj complains to Howard and Bernadette that Anu broke up with him over one mistake. Howard tells Raj that he got out of that relationship in time. Bernadette doesn't want Howard to diss Anu like Penny did to him when they broke up. Also Howard should keep lying to Raj. Raj agrees with Anu that he may have only agreed to the wedding because of the martial status of his friends.

On Zack's boat, Leonard and Penny meet his wife Marissa who is as dull-witted as Zack. They met in a bookstore where she worked which Marissa thinks is a really funny story. According to Zack she is super-smart because she can tell you if you identify any book and she can let you know if she has heard of it. Poor Zack bumps his head on the door every time he goes downstairs though he hardly notices it any more. Leonard and Penny have a touching moment when it turns out that both of them are actually jealous of the success Zack has. The Hofstadters want to be jealous of Zack and his success, but he is so sweet, they both just want him to be happy. After the Johnsons bring over the wine they ask Leonard be a sperm donor since they can't have kids. Leonard is the smartest and nicest person they know. Zack and his frat brothers used to play a crotch kicking games that left him sterile or maybe not. Penny is shocked, doesn't even want the subject brought up and thinks that it is a weird request. And she wonders it the not-so-bright couple should even be parents.

Raj receives a call from his father who is upset about the couple are calling off the wedding and that he inadvertently spied on Anu. Also, he wants the blame put on Anu so that he can get his wedding deposit back. Raj still wants to marry Anu, but he feels that she no longer wants to. Dr. K thinks that Raj shouldn't be so hard on himself. He still calls him a dope, but that his heart is in the right place.

Talking to her Posse, Penny mentions the dumb smile on Leonard's face as he was asked to be the father of their baby. Bernadette thinks that's its flattering while Penny thinks that its creepy. Penny is also dismayed that Leonard would choose to do this because he wants children while she doesn't. The two scientists (Bernadette and Amy) take Leonard's side, telling Penny that there is a strong drive to pass on one's genetic material, though as her husband, Leonard really stinks.

Raj apologizes "Love Actually"-style.

Leonard discusses the topic with Sheldon and he just wonders why Zack and Marissa wanted to choose him. Leonard is annoyed since the Johnsons think that he was nice and smart and he can't see why Penny is freaking out. Sheldon commented that Penny might object to there being a child out there that was biologically his, but not hers. Leonard agrees that he may have a point.

Raj returns to Anu's apartment. He admits that he was marrying her for the wrong reasons and that she deserves someone who knows how amazing she was. He wants to be that man. Then Raj gets her to agree to start over at the beginning by dating each other.

Penny returns home and Leonard tells her that he won't help out Zack and Marissa if she doesn't want him to. This surprises Penny. She leaves and gets Leonard a sample cup telling him that he can donate his genetic material if he wants to. She didn't really give him a choice in her decision to not have kids, so she doesn't think she can stop him from doing this. Penny tells him that Zack couldn't have picked a better person. Leonard thanks her and picks up the phone. Instead of calling Zack, he is calling Sheldon and wants Penny to repeat her last statement to his condescending friend.



  • Title Reference: The title refers to Zack Johnson's request to Leonard to be a sperm donor.
  • Taping date: December 11, 2018
  • This episode was watched by 13.53 million people with a rating of 2.4 (adults 18-49).
  • Total viewers including DVR users 18.01 million.
  • This episode aired in Canada on January 10, 2019.
  • Chuck Lorre's vanity card. [1]


  • Caroline Preece at Den of Geek! - It also once again throws doubt on what the heck the show is going to do in its finale. It was confusing for Raj to announce a Valentine's Day wedding date since he removed the nuptials from the running, and Penny also took a pregnancy off the table. Or did she? Zack's request for Leonard to be his and his wife's sperm donor is one of the weirdest storylines the show has ever done and yet feels like the perfect thing for these last few episodes to grapple with. There's huge potential for humour, if not just because Zack is hilarious and an easy nostalgia point, and it puts Penny and Leonard back in the spotlight for a change. It's bizarre, but I love it. [2]
  • IMDb user review [3]


  • Half-way through the last season.
  • Zack's request could push Penny enough to want kids with Leonard since another couple wants him to be the father of their kids.
  • Audiences will get to meet Zack's new wife Marissa for the first time. It was revealed in "The Cognition Regeneration" (S10E22) that he was engaged to a woman named Sarah, but it now appears that their engagement broke off.
  • Penny's Posse is having drinks in the same bar from season 4 when Amy first met Zack and experienced her first sexual arousal ("The Alien Parasite Hypothesis").
  • Zack's wife Marissa is no smarter than he is.
  • This episode once again highlights how rude Sheldon is to Leonard, as whenever Leonard mentions how smart and hard-working he is, or that he's contributed a lot towards science, Sheldon makes a face indicating that he disagrees with Leonard, no matter how true the claims really are.
  • Leonard decides to help Zack and Marissa out and donate sperm to have their kid, since it would mean he'd get to be a father.
  • Raj and Anu got back together and decide to put the wedding on hold for now before Raj messes this up again.
  • The concept of sperm donation was part of the very first scene of TBBT pilot. Leonard and Sheldon almost donated sperm once in the pilot to make some money for dinner, but ultimately didn't.


Leonard: So are you saying you’re okay with this?
Penny: I don’t know, but when I told you I didn’t want kids you didn’t have a say in that so maybe I don’t need to have a say in this.
Leonard: Thank you.
Penny: They couldn’t have picked a better person.




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