The Irish Pub Formulation

The Irish Pub Formulation is the sixth episode of the fourth season of the CBS sitcom The Big Bang Theory. It aired on October 28th, 2010.

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 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 mixed in his food to Leonard. The episode ends with Sheldon preparing snow cones and serving one to Leonard and Sheldon confesses they were made with mango caterpillar, Leonard then spits out the snow cone and leaves the apartment upset.

Trivia

 * Penny does not appear nor is mentioned in this episode, as the writers wrote her out of this and the previous episode, after Kaley Cuoco, the actress who plays Penny, broke her leg in a horseback riding incident.
 * Thomas the Tank Engine was shown in Penny's apartment before but this is the first time it is referenced in the show.
 * Sheldon has a thermodynamic fluctuations seminar the day after Priya spent the night with Leonard.
 * This episode introduces Priya Koothrappali and her dating Leonard.
 * In the car, Leonard says, "Okay fine, I’m, I’m a horrible human being, I’m the Darth Vader of Pasadena." He was compared to Darth Vader while likewise driving the car, in The Large Hadron Collision.
 * Sheldon retorts, "You’re far too short to be Darth Vader. At best you might be a turncoat Ewok." Darth Vader is 6'8" (2.03 m), an Ewok is about 3'3" (1 m), and Leonard is 5'5" (1.65 m).


 * Leonard is 1/32nd Cherokee.
 * Sheldon tells Raj, "But that behavior is beyond the pale and cannot be tolerated. We are no longer friends." In The Friendship Algorithm, Sheldon similarly remarked to Raj, "Maintaining five friendships promises to be a Herculean task. So, I’m going to have to let one of you go. ... Raj, you’re out." In both instances, he almost immediately rescinded the decision.