Sex mit der Erzfeindin

When the four nerds come back from the Renaissance festival, that to Sheldon's eyes has been "the worst Renaissance festival ever", they meet their neighbor Penny in the hallway, who introduces her new boyfriend Eric to the four scientists. One can see that Leonard is not enthusiastic about that.

A few days later Leonard is approached by Leslie in the Caltech Institute. She has heard that the "relationship" between Leonard and Penny has suffered a harsh setback. Considering the new situation Leslie suggests a relationship. No sooner said than done. The two promptly arrange a dinner date at Leonard's and Sheldon's apartment. Since Sheldon is not particularly fond of Leslie and in order to enjoy the beginning of the relationship in a private atmosphere, Leonard asks his room mate to spend the evening outside of the apartment. Even though Sheldon is not delighted about that, he does his friend a favor and looks for a quiet place in the staircase with his laptop.

At dinner, Leslie quickly gets to the topic of family and children.. Furthermore, she reveals to Leonard the she will not yet have sex with him because she wants a long term and serious relationship. Over and over again, in the course of the evening they are disturbed by Sheldon because he needs to use the toilet or the battery of his laptop dies.

As the evening comes to an end, Leonard accompanies his new girlfriend to the door. There they meet Penny and Eric who are also saying good bye to each other. No wonder that the goodbye kiss becomes a sort of competition between the two couples, or more specifically,  between Leonard and Penny.

When a few days later Leonard has another date with Sheldon's arch enemy, Sheldon escapes into the stairwell again. When Penny bumps into him there, he begs her to get back together with Leonard since he would clearly prefer her rather than Leslie Winkle. But Penny's view is that she will only be friends with Leonard, which Sheldon deeply regrets.

Later on a dispute arises in Apartment 4A between Sheldon, the advocate of string theory and Leslie, a supporter of loop quantum gravity. When Leslie realizes from this argument that Leonard believes in a different theory of  quantum mechanics than she herself, she leaves the apartment in anger.