Penny wants to throw a birthday party for Leonard since he never had one in his childhood and after the failed one years ago.
Everyone is in Apartment 4A and eating and the gang discuss the fact that Leonard's birthday is coming up soon. Leonard tells everyone that he never celebrated his birthday as a child due to his parents's wishes to study the holidays for their "anthropological and sociological implications on society" but tells everyone the blame falls mostly on his mother, Beverly something which everyone but Sheldon deems unfair. Howard and Bernadette declares what a bitch Beverly is and Sheldon tells them off, but Bernadette manages to make Sheldon shut up because he's just defending her since he's the only one who likes Leonard's mother while almost no one else in the world does. Penny decides she wants to plan another birthday party for her beloved husband.
The next day, Leonard and the girls plan his party. He says he would like a chocolate cake, balloons, decorations and some music and gives the guys a list of who to invite and they contact them all to see if they can come. In Apartment 4B, Leonard and Penny are getting ready for the party and Leonard tells her that he's so happy she's doing this for him.
Later, at the party, everyone is busy with each other and talking. The guests include Mary Cooper, Wil Wheaton, Dr. Gallo, Dave Gibbs, Leslie Winkle, Barry Kripke, Penny's parents, sister, brother, brother-in-law and nephew, Meemaw and Mandy Chao who are all on Leonard's list, but another guest who is at the party and was not on Leonard's list: Beverly. She is talking to Sheldon and they are together talking.
Soon, everyone gets ready to surprise Leonard for his party. Alex goes to Apartment 4B and tells them to come by in 2 minutes. She retreats back to Apartment 4A and signals everyone to get ready. Once the 2 minutes finish, Leonard and Penny head over to Apartment 4A and once they open the door and walk inside, everyone shouts "Surprise!". Leonard and Penny are amazed, but the amazement turns into disgust when Leonard and Penny see Beverly. Leonard furiously demands from Sheldon, Howard, Raj and Stuart what his mother is doing at the party and Sheldon reveals that he secretly invited her, something which also makes Howard, Raj and Stuart, as well as Penny, Bernadette, Amy, Emily and Alex disgruntled.
Sheldon tells everyone that the party and everyone begins spending their time. Bernadette takes Leonard and Penny to a corner and tells Leonard just to remain calm and enjoy his party and if his mother does something that angers him, she'll kick her butt which Leonard laughs at and says "Okay.". She also confesses that she knew Sheldon was doing this and he made her not to tell him, which gets Leonard a little more upset, but seems to think better of it. Penny tells him to relax and she gives him a kiss on his left cheek. Meanwhile Sheldon and Amy are talking with his mother and Meemaw. They were glad to come to Leonard's party, knowing that he is such a wonderful boy.
Later, everyone does cake, sings 'Happy Birthday' to Leonard and he blows the candle out while everyone applauds. The gang and a few other guests (not everybody) have some speeches to give to Leonard.
Bernadette, Raj, Alex, Stuart and Sheldon say that Leonard is a good guy and they are all happy to have him in their lives. Amy says that Leonard is a very good guy and that since she had no friends in her childhood and teenage years, he, like the rest of the gang, has made her life happier by being her friend and him expressing platonic love in her life, like the rest of the gang. Howard tells Leonard that he is a very good guy and he is very happy to have Leonard as a friend and he wants Leonard to know that Leonard has also made Howard's life happier and better and that he will always hold his relationship and marriage with Penny in high regard and Howard is about to say how and why: Before Howard met Bernadette, he was a creepy pervert and a lonely nerd living with his mother and later the two guys made a Girlfriend Pact where when one of them gets a girlfriend and set the single guy up with a female friend of that guy's girlfriend and then Leonard told Penny about it and she accepted and then she chose Bernadette and during their dinner date that night, Howard and Bernadette didn't have much of anything in common, but then they felt a connection when they found out they both have overprotective mothers, and overtime, Howard changed and Bernadette made him a better man. And this was all through Leonard and Penny's machinations. And if it hadn't been for all this, who knows how long it would've been for Howard to become a better person? Penny's parents tell Leonard that they are very happy for Penny to have him, that they are very fond of him and are happy to have him as a son-in-law. Even Penny's sister, brother, brother-in-law and nephew express their admiration for the sweet and clever Leonard Hofstadter. Mary gives a touching speech on what a good guy he is and how he has been able to put up with Sheldon for all those years. Finally, Penny gives a speech to everyone saying that Leonard is a really good guy and a sweet and clever man who is loyal and would never cheat on his own accord and that Leonard is the love of her life and will always stay with him, happily married. Leonard thanks everyone for their speeches, but Beverly butts in saying she wants to say something too, but Leonard forbids her, but Sheldon tells him to be quiet and to let his mother talk. She just says "Happy Birthday, sissy pants." Sheldon gives a chuckle, but nobody else is happy. Bernadette tells everyone to pay attention to her. She tells Leonard she has a wonderful present for him. Leonard wonders what it could be and soon Bernadette shows that it's throwing water right into Beverly's face! Everybody except Sheldon laughs. Bernadette says that will teach Beverly a good lesson for never letting Leonard celebrate his birthday or Christmas as a child and making his childhood difficult and being a bad mother to him. Dr. Gallo shouts that all of Beverly's books are food for incinerators and Penny's parents, Mary and Meemaw yell out that while she doesn't appreciate Leonard, they do. And suddenly, Howard blows a whistle and out from a trunk jumps Bernadette's father Mike and her mother. They both approach Beverly, grab her, handcuff her and tell everyone that they're gonna have her spend time in the criminal detention cell in their house and they drag her away. Sheldon tries to stop Bernadette's parents, but he is held back.
Leonard comments "This is the best birthday ever."