Leonard's father visits, meets the gang, pisses Leonard off with the past and does not take much of a shine to Penny.
The gang is in Apartment 4A, reading, playing games, listening to music and singing while Leonard is on the phone. He soon hangs up and the rest of the gang looks at him, confused. Leonard tells them that his father is coming to visit.
The next day, everyone is back at 4A, watching TV. Suddenly, there is a knock on the door. Leonard goes to answer it and opens it to find his father standing outside in the hallway. They greet each other and Leonard invites him in. Leonard introduces his father to all of his friends. When he gets to Penny, he is surprised to find out that she is married to his son and he also sees how in love they are.
Later, Leonard makes his father tea and starts a conversation with everyone. Dr. Hofstadter tells Leonard that he has bought a picture of a skeleton which bums Leonard out, as well as the gang and then goes on to tell about Leonard's past, which upsets Leonard. The boys all launch into laughing while all 5 girls get angry. Penny, Bernadette and Amy speak up and berate him for not being so affectionate to Leonard in his childhood, Emily gives him a whack in the arm and Alex blows into his face. They then reprimand the guys for not helping Leonard through his trouble since his childhood was difficult and Penny comforts Leonard.
Leonard, Sheldon, Howard and Raj later take Dr. Hofstadter to Caltech so he can see the university. He is taken to his son's laboratory where he sees a new experiment Leonard is conducting, and after seeing the success, Leonard's father seems happy. He also visits Sheldon's office, Howard's engineering laboratory and Raj's office, where he is not so amused with Sheldon's work, is fascinated by Howard's machines and tells Raj that he loves telescopes and enjoys looking at outer space through them.
Dr. Hofstadter soon picks on Penny briefly when they are all back at 4A for dinner. Leonard tells him to stop because his attitude is rude. He also goes on to say that he is the right man for Penny, because he previous guys were guys who were either not so successful in their careers, cheaters and sex life exposers, while Leonard is none of that. Even the rest of the gang are okay with Leonard and Penny together. Leonard tells his father that he has loved Penny since the moment they met and he decides to keep things the way Leonard wants them.
But then, Dr. Hofstadter admits that he was feeling jealous of Leonard's happy marriage to Penny. He had not been in a good marriage with Leonard's mother for years and he cheated on her because he had had enough of the lack of love and intimacy. Sheldon remarks that Leonard's mother is a brilliant and remarkable woman, but Bernadette tells him to shut up. Penny advises him, that there are lots of women out there and that he should try to pick one up, either in a bar or a dating website, but everyone agrees to help him find a woman. Dr. Hofstadter also feels remorseful about his relationship with Leonard. He apologizes to Leonard and the girls and he forgives the girls for what they did and the boys apologize to Leonard for their behavior the previous night. Dr. Hofstadter hugs Leonard and Penny and he accepts the latter, despite knowing that she has a lack of academic knowledge.
The next day, Leonard's father leaves to return to New Jersey. He gets into a cab and sets off for the airport while everyone waves goodbye.