Brilliant physicist roommates Leonard and Sheldon meet their new neighbour Penny. Leonard is attracted to her and asks her over for lunch to welcome her to the building. He is so kind to her that he and Sheldon end up trying to retrieve her TV from her ex-boyfriend. While they are out she is entertained by their friends, sleazy Howard and silent Rajesh.

Penny asks Leonard to sign for a piece furniture while she is out. But when he puts it into her apartment, Sheldon sees what a mess it is and insists on tidying up. Penny returns soon after and so Sheldon breaks in at night to tidy.

Leonard sees Penny with another guy and asks out a girl he works with. She rejects him and he begins to become depressed. He tries to ask out Penny but she doesn’t realise that it is a date. She tells him that she is not ready to date someone new and he doesn’t admit that it was a date.

Sheldon offends the new department head at the University and gets fired. He begins adopting a new obsession each day and refuses to leave the apartment. Leonard calls Sheldon’s mother who comes to stay. She forces him to go and get his job back.

Leslie Winkle seduces Leonard but only wants a one night stand. Leonard is fixated on what Penny thinks of it all.

Penny invites the guys to her Halloween party. They all dress up in elaborate costumes and head there early but have trouble meeting new people. Leonard decides to try and match wits with Penny’s ex-boyfriend and they end up arguing. Leonard leaves upset and embarrassed. Penny comes to see if he is ok and ends up drunkenly kissing him.

Penny’s friend Christy begins hooking up with Wolowitz and it leaves the guys one short for all their regular activities, much to Sheldon’s despair.

Rajesh's parents set him up on a blind date and he fears he won’t be able to talk to her. Penny needs to practise her bar tending skills for work and once Rajesh drinks he can talk to her. On the date with Lalita he is an obnoxious drunk and she is all too happy to leave with Sheldon when he takes an interest in her.

Sheldon and Leonard are invited to present their paper on Super Solids to a physics conference but Sheldon doesn’t want to. Leonard does and they fall out over it. When Leonard goes without him Sheldon shows up and starts a fight.

Leonard hears Penny singing and it is not good. When she asks him to come to a showcase he lies and says that he and Sheldon have plans. Sheldon is uncomfortable with the lie which Leonard gives and so comes up with a new one. Each time Penny believes him he finds new holes in his own lie and new ways to patch it up.

When Sheldon becomes ill Leonard runs away. Howard and Rajesh join him in hiding because of how needy Sheldon becomes. Penny ends up having to look after him.

Dennis Kim, a child genius is looking around the university and seems to have outstripped Sheldon in every way. Sheldon decides to give up his own work and focus on other tasks. These annoy the guys so much that they try to distract Dennis in order to get Sheldon back to normal.

It’s the annual Physics Bowl and the guys boot Sheldon off their team because he is being a pain. They recruit Leslie Winkle and Sheldon takes them all on alone.

Leonard accidentally buys a full size prop of the time machine from the movie The Time Machine. As the guys attempt to bring it up to their apartment they block the stairs and Penny ends up missing work. She scolds Leonard for having all these toys and so he tries to get rid of them all. But Sheldon points out Penny’s hypocrisy and she apologises.

Sheldon’s twin sister comes to visit and all the guys start hitting on her.

When Penny finds out that Leonard has never had a birthday party she decides to throw his a surprise one. While she takes Sheldon to buy him a present, Howard has to distract Leonard.

After another messy break up Penny asks Leonard why she can’t go out with a nice honest guy. So he asks her out. She says yes but they both feel nervous about it and go to Sheldon for advice. He has his own problems though as he is learning Mandarin in order to properly accuse his local Chinese Restaurant of serving his orange chicken instead of tangerine chicken.