The Big Bang Theory Season 4 Episode 6
The Irish Pub Formulation
Raj's sister comes into town and Leonard and Priya (Aarti Mann), resume a secret affair they began five years ago. They try to keep it a secret from Raj, however, this time around, Sheldon finds out when he stays up past his bedtime to play computer games. Leonard begs Sheldon to keep a secret, but the only way he can is through an elaborate alibi involving a red-headed waitress from an Irish Pub. Leonard tries to go along with it, but eventually chokes under the silliness of the alibi and comes clean. The guys all forgive each other for the various pranks they've pulled on each other.