Who says Sheldon doesn't know how to have fun? He even invented his own board game: Research Lab ("The Physics is Theoretical, But the Fun is Real!") Sheldon is more than happy to use Penny and Leonard as beta testers and think they'll wipe the floor with Raj and Howard on Sunday, but Penny has to pick up her friend Justin from the airport. Leonard is happy with this excuse and is happy to join her. However, Justin has a lot of stuff, such as guitars and amps, and he will be crashing on her couch for a couple of weeks while he finds some recording studio time. All of this comes as a bit of a surprise to Leonard, who did not know about any of it. He starts questioning Penny about how close of a friend he is. Also, how straight or gay of one he is. Penny says they went out once or twice but were never serious. Leonard, channeling his inner Ross Geller, automatically assumes this means they've seen each other naked and doesn't want him crashing on her couch. She takes serious offense to his directive, thinking he's treating her like an idiot. Meantime, Sheldon is desperately trying to get them back to playing the game. Or to be more precise, to stop arguing. However, the fight is on.
LEONARD: If you want to invite some stupid guitarist to sleep on your couch, fine. Why don't you just rent a couple of bunk beds and invite the Black-Eyed Peas!
PENNY: Hey if I want to invite the entire lineup of Lollapalooza to sleep at my apartment, I will. And it's none of your business!
LEONARD: Do you know how childish you sound right now!
PENNY: Oh, at least I'm not a child now instead of stupid...WHAT THE HELL ARE YOU DOING, SHELDON?
Sheldon was apparently running the blender to drown out their argument. Penny gives up trying to talk over the blender and leaves. On the bright side, Leonard won the first game of "Research Lab," and Sheldon gives him a celebratory sno-cone.
Leonard doesn't appear to be letting this go, and Sheldon doesn't appear to be getting used to people fighting. As Leonard complains about the argument in the car, Sheldon tries to get him to play "Scientist," where you put three scientists in the order of their contributions to their respective field. ("Ready to have some fun?") But Leonard is going out of his mind. He thinks even if he said it was cool to Penny, she'd rag him for even giving his approval, and she'd be doing him a favor just by being in the same room as him. Of course, he's having this whole argument with himself. And now we know how to get Sheldon to walk to wherever he needs to go.
It gets worse. On the way to a movie (where they only have 17 minutes to get there, thus having to skip the concession stand and the pre-show urination), the guys run into Penny, who hopes Leonard is ready to apologize. Naturally, he isn't. (Are they really showing a digital reprint of "Time Bandits?" That would be so cool.) She reads him the riot act again in front of Howard and Raj, but Sheldon successfully ducked out in time.
RAJ: That was unpleasant.
HOWARD: I don't need my pre-show urination now.
To kill time before the next showing, they head to the comic book store, which has replaced Stuart with Gunther from "Friends," although he still apparently pines after blonde waitresses. Leonard asks Howard if he thinks Penny is out of line.
HOWARD: Of course she was out of line, but she'll have a new boyfriend by this time tomorrow, and you'll have a new girlfriend in the time it takes you to build one. The only question is how long before you fold?
Leonard refuses to fold, and Raj thinks Leonard is wrong. As an example, he references the time he took a cooking class with Howard, but Howard blew him off to do a liquid cleansing diet with his mother.
RAJ: I had to cook a chicken and rice dish with a Vegan? Do you know what Vegan chicken and rice is? RICE!
HOWARD: Do you think I was having a fun time listening to my mother say 'have you ever peed so much'?!?!?
RAJ: YOU ARE SUCH A MOMMA'S BOY!
HOWARD: DON'T BRING MY MOTHER INTO THIS!
You can guess what is happening to Sheldon at this point.
SHELDON: STOP IT, BOTH OF YOU! ALL OF THIS FIGHTING, I MIGHT AS WELL BE BACK WITH MY PARENTS!! 'GEORGE, I TOLD YOU IF YOU DIDN'T QUIT DRINKING, I'D LEAVE YOU!' 'WELL, I GUESS THAT MAKES YOU A LIAR BECAUSE YOU'RE STILL HERE!' 'STOP YELLING! YOU'RE MAKING SHELDON CRY!''I TELL YOU WHAT'S MAKIN' SHELDON CRY! THAT I LET YOU NAME HIM SHELDON!'"
It's time for Sheldon to take action. He stops at the Cheesecake Factory under the guise of having lunch to ask Penny to apologize to Leonard. She's not having it, so Sheldon slowly orders lunch to make her stay. He tries to point out the myriad of things that Leonard does not say about her, such as her driving, the bed full of stuffed animals that stare at him during their "amorous activities," her tardiness, or her singing. (actually, that's on Sheldon's list) However, this is all new information to her, and it only serves to display her bad temper, which Sheldon also mentions as something Leonard never comments on.
PENNY: Well, you can tell Leonard that he can drop dead!
SHELDON: And she wonders why she's undertipped.
Sheldon quickly goes home and suggests to Leonard to apologize to Penny before she busts down the door. Penny comes in at full boil to yell at Leonard for hating her singing, her stuffed animals, and her driving. This time, the yelling is too much for Sheldon. He runs away. First he tries Raj's place, but Raj is having an argument with his parents for not coming to New Delhi for Cousin Sanjay's wedding where they will set him up on a date. (She just got her stomach stapled and her self-esteem hasn't gone up yet.) They're upset because the closest thing they have to a daughter-in-law is Howard. At Howard's, he and his mother are having an argument (which you can hear all the way outside, natch) about who should get the door.
After driving around in Penny's Deathmobile (apparently, her driving is dangerous), they find Sheldon at the comic book store, where he has imagined himself an alternate universe where Leonard and Penny don't exist. (That, or he's ignoring them.) Their fight seems ready to continue, but they realize what they're doing to Sheldon and try to talk him out of it. They try to convince him they're done fighting.
SHELDON: I've heard that before. The next thing you know I'm in my bedroom blaring a Richard Feynman lecture while my mother is shouting that Jesus would forgive her for putting ground glass in my father's meatloaf. And my dad is on the roof skeet-shooting her Franklin Mint collectable plates.
Unfortunately, there's still a bit of hostility between Leonard and Penny because he can't see why Justin can't crash somewhere else, and she can't see why it's bothering him so much. Sheldon activates a robot to drown them out. But they tell him that he has to get past his childhood issues. So he drowns them out with another robot. Penny tries to convince him that they're in a relationship and they will sometimes fight, but they will always love him.
PENNY: We'll buy you this robot.
SHELDON: I want that one.
LEONARD: We're not going to buy him a robot. He'll just play with it twice and it will end up in his closet.
PENNY: Buy him the robot.
SHELDON: Can I have this comic book, too?
However, Sheldon probably should have demanded more. Penny and Leonard found a couch for Justin to sleep on: Sheldon's couch. (At least Justin isn't sleeping in Sheldon's spot.)