Season 3

Penny is in her apartment and is painting her nails, when she hears an argument between Leonard and Sheldon. They are fighting because Leonard wants to put the Thermostat up 2 degrees. Then Leonard knocks at Penny's and asks her for a room for the night.

The conversation comes to how Leonard and Sheldon got to know each other, and Leonard tells her the story

Shortly after Leonard started the university, he met Sheldon, who was looking for a roommate. Already at the door, he is warned by the previous tenant to run away fast. But Leonard ignored the warning. Leonard rode the elevator to Sheldon's apartment. When he got to know Sheldon, he asked him scientific questions, so he will be allowed in the apartment. The apartment is clearly empty, and they find only two garden chairs and a television in the living room. Sheldon explains to him right away, that the place will always be his. Sheldon fills in a form with Leonard, and shows him the remaining room. Leonard makes a couple of jokes, but Sheldon don't get them.

Together they set up a roommate contract.

Penny cannot understand why Leonard had admitted that about himself But Leonard wants to tell her how it was, when he brought a woman.

Sheldon knocks on Leonard's door, and he is lying with a woman in bed. He interrupts them both, and gives Leonard a lecture about disregarding the contract on the subject of visitors. Penny is shocked when she hears this. But Leonard finds that to be okay, because his girlfriend was exposed as a North Korean Spy.

Leonard wants her to tell how the elevator was destroyed:

When Sheldon comes home, Leonard is playing video games with Raj and Howard. They brought a couch with them. Sheldon is shocked by the strange people and furniture. But Leonard is able to calm him down.

Sheldon sits with the others and gets to know Howard and Raj better. But Sheldon is so unfriendly that the other three leave.

They go to Howard's, where they are greeted loudly by his mother. In his room, they examine the three-stage rocket he made. Leonard offers him his fuel, which he had in the apartment. They experiment with the fuel in the kitchen with explosives in the water system. This should have caused a great boost. Sheldon warns Leonard that the amounts were not correctly calculated and that he worked with models and not real rockets. However, Leonard ignored him. The fuel begins to emit strong smoke and the four begin to panic. They place the bottle in the elevator and want to bring it down. However, just as Leonard enters the elevator with the bottle, Sheldon comes and pulls him out of it. The bottle begins to go down alone, and then there is a huge explosion.

As Penny hears, she gets angry because, thanks to Leonard, she must walk up the stairs every day. Leonard thinks that he not only owes his life to Sheldon, but because he hasn't been ratted out as well.

Leonard comes back to the apartment and apologizes to Sheldon. The two of them get along once again- until Leonard turns on the television to Babylon 5. Immediately, you hear a loud argument in the apartment once again.