Season 4, Episode 6 | October 28th, 2010
Koothrappali’s sister, Priya, comes for a visit and we find out she and Leonard have a romantic past unbeknownst to the rest of the guys because Leonard and Howard made a pact to never make a move on her. When Sheldon discovers Leonard sneaking Priya out of the apartment the next morning, Leonard bribes him with the promise of having a train day if Sheldon will keep the secret. Sheldon agrees but is terrible at keeping secrets so he instead comes up with an elaborate story of how Leonard met an Irish bartendar at Pasadena's best Irish pub. Leonard tells Sheldon that the story isn't necessary and that if he just keeps his mouth shut, everything will be okay.
When the guys gather the next day, after Koothrappli has taken Priya to the airport, Leonard feels so guilty about what he did that he immediately starts telling the story that Sheldon concoted for him. The truth comes out and both Raj and Howard feel betrayed. This starts an argument with all of them which in turn reveals secrets they've all been keeping from each other.