The Big Bang Theory Season 5 Episode 13 : The Recombination Hypothesis
Upon arriving home, Sheldon discovers that the cutout of Mr. Spock he ordered is of Zachary Quinto, not Leonard Nimoy. As he gripes about this, Leonard notices Penny across the hall, and spontaneously asks her out on a date. Their dinner starts well, but soon goes sour when Leonard insists on over-analyzing their relationship. He comes home in a bad mood, causing Howard and Raj to assume he and Penny are on the outs. However, that night, Penny invites him over via text, and, after warning him not to talk, they have sex. They agree to keep their relationship secret. When everyone has dinner at the Cheesecake Factory, Leonard overplays his fake annoyance, genuinely angering Penny. They have a late-night argument, followed by more sex. Leonard tells Penny that he cannot imagine this ending well.