The Irish Pub Formulation

'''The Irish Pub Formulation is the sixth episode of the fourth season of the Big Bang Theory.

Plot Summary
Leonard hides from his friends that he's having a secret affair with Raj's sister, Priya.

Expanded Plot
Raj's sister Priya (Aarti Mann) visits and inadvertently causes trouble for the friends. She and Leonard apparently have a history, despite Leonard's pinky swear with Howard that they would not interact with her in deference to Raj. They hook up, but are discovered by Sheldon, who devises an elaborate lie to cover it up. Leonard cannot persist in the deception and reveals the assignation. The guys then unleash confessions of all of the deceptions they have perpetrated on each other, including Sheldon's experiment feeding moth parts to Leonard. The episode ends with Sheldon preparing snow cones made with mango caterpillar.

Trivia

 * Penny does not appear nor is mentioned in this episode, as the writers wrote her out of this and the next episode, after Kaley Cuoco, the actress who plays Penny, broke her leg in a horseback riding incident.
 * Title reference to the Irish pub that Sheldon included in the details of the deception to cover up Leonard's sleeping with Priya.