The Recombination Hypothesis
When Leonard and Sheldon arrive home Leonard sees Penny through her open door and asks her out, who after initial hesitation, accepts. Their date progresses smoothly until he asks her whether they will get back together. They get into a big argument, following which Leonard returns home in a huff. Later that night, Penny calls Leonard and asks him to come out of his apartment. As he does this, Penny kisses him full on the mouth and they end up having sex in her apartment. After this, they decide to pursue their relationship once again, but also decide to take things slow and pretend they are still single. Leonard finds it difficult to keep the relationship secret; when he returns to his apartment at 3 am, he is forced to lie to Sheldon (who saw him entering the apartment); the next night at the Cheesecake factory, he overplays his fake annoyance over Penny, following which the duo again engage in a heated argument. But later that night, they again have sex. After this, he asks her what they are doing, since every scenario that he plays out about them ends badly, to which she responds that it is because he over-thinks things. It is then revealed that Leonard daydreamed the entire episode. He is still outside his apartment with Sheldon. Realizing he does over-think things, he goes over to Penny’s apartment and asks her out. After initial hesitation, she accepts. The episode ends with Penny preparing for her date. She imagines a scene where she marries Leonard, but is heavily pregnant, prompting her to decide to go to the drug store before the date.
Title reference: Leonard contemplating the ramifications of asking Penny out on a date.