Penny tries to seduce an abstaining Leonard to ruin his "donation" for her ex-boyfriend, Zack, and his wife, Marissa. Also, Howard, Bernadette, Anu and Raj turn Koothrappali's canceled bachelor party into a couple’s trip aboard the "Vomit Comet".
Zack and Marissa are visiting Leonard and Penny discussing Leonard’s pending fatherly donation. They are both grateful for Leonard’s help. Penny still thinks the subject is weird. The Johnsons' also think that it is weird that their kid will look like Leonard. Zack adds that if it’s a girl, it’ll look like Penny. No. Typical Zack. Leonard needs to abstain from Penny to provide the most potent sample on Monday. They would like to have a son, so Zack wants Leonard to keep thinking “Little boys. Little boys!” Sheldon wanders in on the pretext to borrow a book and mentions how tall and smart he is. Amy runs in and drags him home because Zack and Marissa want them to have Leonard’s baby and not his. As Sheldon leaves has also throws in his blue eyes, no cavities in his teeth and not being lactose intolerant.Raj is visiting Howard and is lamenting his wedding that would have been that week, though at least he is still with Anu. Howard adds the $3000 in airline tickets to India that he can’t use. There is also the incredible bachelor party that Howard had planned. Through NASA he got four tickets on the “Vomit Comet” allowing them to feel weightlessness. Raj is excited and suggests a guys' weekend just like in the good old days. Howard adds that they were sad, desperate and totally alone then.
At the cafeteria Howard announces that they can all go on the "Vomit Comet" together. Sheldon is not interested in vomiting, while Penny’s father Wyatt is visiting the Hofstadters' over the weekend. Raj suggests then making it a couples’ weekend and they could bring Bernadette and Anu. Howard plans to ask Stuart or Bert since he doesn’t think that Bernie would like to do that.
Penny comes into the kitchen and finds Leonard taking some vitamins to help his virility. When Wyatt knocks on the door Penny asks Leonard not to bring up the subject of their kids, Zack and Marisa’s project or the Cornhuskers. Wyatt comes in calling Penny “Slugger” and complimenting Leonard on his strong handshake. Leonard has been taking vitamins. Penny offers her dad a beer. He replies that he’ll one if she is since he knows that she’s not pregnant. Penny then blurts out about the Leonard-Zack project to take the subject off of their non-existent kids.
Howard enters the kitchen and asks if Bernie minds if he goes away that weekend with Raj. The two were taking Anu and Stuart which bothers Bernadette that she wasn’t asked. Howard insists that she wouldn’t want to float around weightless since she got sick while in line to the Tea Cups ride at Disneyland. First Howard says that he doesn’t want her to make everyone else miserable, but realizing what he said he invites her to go with them.Back at the Hofstadters’ place, Leonard thanks Penny for throwing him under the bus. Wyatt wants to know why Leonard is okay with not wanting a baby with Penny, but wanting to have one for Penny’s dumb ex-boyfriend. Penny supports Leonard, but is still okay with her dad making a scene about it. Leonard calmly explains that Zack and his wife can't have their own children, and likes that Leonard's a smart and caring person, traits that they want for their kid. Wyatt then mentions about hiring out his prize stallion for stud purposes. Penny retorts that their situation is not like that, though Leonard is fine being compared with a prize stud.
Leonard comes to bed talking about how much Wyatt likes him since Penny used to date idiots. Penny does admit that she did marry a stud. Penny wants to make it up to him after throwing him to the lions earlier. She forgot that Leonard still had three days of abstinence ahead. Penny then comes to bed wearing a sexy outfit, snuggles up to Leonard and seductively starts to read her comic book. “Bam! Pow! Take that Batman!”
The two couples are driving in the car and Anu is so glad that Bernadette could come especially since Howard claimed that it wasn’t her thing. Bernadette snaps saying that Howard doesn’t know everything. Anu claims to be an adrenaline junkie doing things like roller coasters and snowboarding. Bernadette counters that she once jumped on the back of her shopping cart and rolled through the meat section. Howard added that just being married to her is a ride.
In Apartment 4A Sheldon comes out into the living room, fixes his breakfast and finds Leonard on their couch. Sheldon calls to Amy who comes in and is also surprised to find Leonard there. He used his spare key to come in and get away from Penny who wanted to have sex. Sheldon comments that he has been there and done that. Amy suggests that Penny still doesn’t want him to succeed. If Penny is objecting, Leonard wants her to tell him. He still wants to leave a little of himself behind in the world. Amy adds that even if that is true, he would not be the kid’s father.
In the airport waiting room, the couples are being briefed for their flight which will be the experience of their life. Bernie tells him that at 90 pounds, she gets weightless if she is holding onto three balloons. While signing the injury and death waivers Bernadette has second thoughts about both of them doing it since they have kids. She was just trying to prove to Howard that he doesn’t know her as well as he thinks he does. And if she does it, she can prove him wrong. Anu thinks that she should just be honest. Bernadette agrees and tells Anu that she owes her $3000 for airline tickets.Penny returns to her apartment and her dad after a long run interrupted by stopping at a diner and a massage chair. Her dad comments that she is not really mad at him and Leonard, but at herself. He says that she regrets her “no baby” plan. Penny agrees that she feels that she is letting everyone down. Wyatt tells her that she could never disappoint him, though her high school years were rough on him. She once stole a horse while drunk. He tells his daughter that whatever she decides he is on her side. Leonard comes marching in and tells Wyatt to back off since it is his and Penny decision. Wyatt has already discussed it with Penny, but compliments Leonard on being forceful. Leonard then tells Penny that he has changed his mind and he’d be heartbroken if there was a kid out there that he couldn’t be the father of. Penny agrees with him and then he asks if her to do that thing from last night; however, Wyatt is still within ear range.
Back in the airport waiting room, everyone is in jumpsuits though Bernadette declines to go. They have kids and she doesn’t think that they should both put themselves in danger. Howard cracks that he didn’t see that coming. Howard suggests that she go so she can prove herself right as the plane flies repeatedly up and down and up and down.
Now floating weightlessly Bernadette is yelling that she hates it so much. Anu says that she is proving her husband wrong. Now she LOVES doing it.
Leonard calls up Zack and Marissa declining to be their sperm donor, but he does suggest that someone else is interested. While on the phone Amy is yelling at Sheldon that he is not going to do that. Sheldon says on the phone that apparently he is not allowed.
- Guest starring:
- Teleplay: Steve Holland, Dave Goetsch & Maria Ferrari
- Story: Bill Prady, Jeremy Howe & Adam Faberman
- Title Reference: The title refers to Penny and Leonard changing their minds about Leonard fathering Zack's baby.
- Taping date: January 22, 2019
- This episode was watched by 13.97 million people with a rating of 2.6 (adults 18-49).
- This episode aired in Canada on February 7, 2019.
- Chuck Lorre's vanity card. 
- Caroline Preece at Den of Geek! - While The Big Bang Theory has been delivering clever twists and turns this season, The Donation Oscillation undermines a lot of that and possibly sheds some goodwill along with it...if Leonard has ultimately decided not to contribute to Zack and his wife’s family, then why has he been agonizing about it for weeks? It’s odd, even if it’s not entirely unexpected. Both story lines figure heavily into this episode, even if only one of them does directly. 
- IMDb user review 
- Penny's father Wyatt reappeared in this episode, marking his fifth appearance in the series.
- Zack and his new wife Marissa also reappear.
- First episode of the last ten episodes.
- Leonard thought about donating sperm for Zack, but changed his mind. Not because of Penny, but for himself. Penny would have respected his wishes whether she likes it or not.
- This episode proved that Penny began to doubt her decision on never having children.
- Zack would have gone to Sheldon since the latter was willing to donate his sperm. Amy told Sheldon no.
- Raj, Howard, Anu and Bernadette were going to Vomit Comet. Bernadette was reluctant to go, but she changed her mind and goes to prove Howard wrong.
- Penny reveals that her sister's name is Lisa.
- Bernadette weighs ninety pounds.
- Penny stole a horse while drunk during her high school years.
- Sheldon and Amy have not had sex for three weeks.
- Raj reveals that his parents had a bad marriage even when he was little.
- Leonard doesn't know who The Cornhuskers are even though he attended Penny's University of Nebraska football party.
- Zack incorrectly thinks that if his wife has a girl through Leonard's donation, she will look like Penny.
- The writers established that her dad called Penny "Slugger" two years before he appeared.
- Penny: Okay, that’s my dad. Now remember, don’t bring up any baby stuff. All right? Not me not wanting one, not you having one with Zack.
- Leonard: Got it.
- Penny: And if he brings it up, just change the subject to literally anything else.
- Leonard: Got it.
- Penny: But not the Cornhuskers, Do not discuss the Cornhuskers.
- Leonard: Is that a sports team?
- Penny: Never mind, you’re good.
- Penny: Can I get you something to drink? Maybe a beer?
- Wyatt: Sure, if you’re having one.
- Penny: Okay.
- Wyatt: And why wouldn’t you, since you’re not pregnant?
- Howard: And I married her which is quite a ride.
- Leonard: Penny, doesn't want to have kids. I can respect that. But this is my chance to leave a part of me behind.
- Amy: Okay, but this isn't going to make you a dad. It's gonna be their baby, not yours.
- Wyatt: I think you’re mad at me and Leonard, but maybe you’re really mad at yourself.
- Penny: Why would I be mad at myself?
- Wyatt: Cause your regret this “no baby” plan of yours.
- Penny: You know it’s pretty crappy of to make me feel bad about this.
- Wyatt: I’m not trying to make you feel bad. But…do you?
- Penny: Yes. I feel like I’m letting everyone down. I hate disappointing Leonard, but I really hate disappointing you.
- Wyatt: Oh, Slugger, you could never disappoint me. Okay, your high school years were a little rough.
- Penny: And I’m sorry about that.
- Wyatt: There was you sneaking out at night and your boyfriends sneaking out in the morning.
- Penny: I said sorry.
- Wyatt: That time you got drunk and stole the horse.
- Penny: Yeah. I was there. I remember. Parts of it.
- Wyatt: But besides all that, being your dad is the best thing ever happen to me.
- Penny: What about Randall and Lisa?
- Wyatt: They’re okay. I just don’t want you to miss out.
- Penny: I know.
- Wyatt: But if this is really what you want, or don’t want, I’m on your side.
- Penny: Thanks.
- Bernadette: [Floating] I hate this so much! Why am I doing this?
- Anu: To prove a stupid point to your husband.
- Bernadette: Oh right. Worth it.
- Leonard: I know someone who might be interested.
- (scene cuts to Apartment 4B)
- Amy: (angry) No. Absolutely not!
- Sheldon: I’m sorry. My wife says I’m not allowed.
|Episodes | Season 12|