Leonard and Priya first met on April 12, 2005. Howard and Leonard made a pinky swear not to hit on her, but it was revealed that Leonard had sexual relations with her, which the other guys did not know about.
Leonard and Priya
They meet each other again in "The Irish Pub Formulation", which is Priya's first on-screen appearance in the series. Leonard and Priya slept together that night, though Priya made clear that her parents prefer her dating an Indian man and does not want them to know about Leonard. Priya leaves the next day, and their relationship temporarily ends.
Later in "The Cohabitation Formulation", the two start dating publicly, much to Raj's disappointment and disapproval. Then in "The Prestidigitation Approximation", Priya starts making requests to Leonard, including wearing contact lenses instead of glasses and wearing "more grown-up clothes". Additionally, Priya requests that Leonard stop hanging out with Penny due to their dating past. This upsets Penny greatly as Leonard is a friend and neighbor, which was why Leonard was very reluctant to break the news to Penny. After a failed attempt at telling her, Penny intrudes on Leonard and Priya during a restaurant date out of jealousy and frustration. She eventually finds out and comes to terms with Priya's demand by the end of the episode, agreeing to respect Leonard's relationship and to keep her distance. In "The Zarnecki Incursion", the relationship becomes rocky when Leonard lies to Priya about working late for two nights, so that he can help Sheldon against a hacker thief who stole his things in World of Warcraft.
Leonard and Priya in the shower, causing a legal showdown with Sheldon
In "The Agreement Dissection", as a result of Priya lawyering Sheldon to defend Leonard, Sheldon retaliates by writing a new Roommate Agreement and forcing Leonard to sign it. Leonard initially refuses, but after Sheldon reveals that he will send an email to Priya's parents about the relationship if he doesn't, Priya freaks out and threatens to break up, forcing Leonard to eventually give in and agree to it. In "The Engagement Reaction", Penny and Priya bond when discussing Leonard's relationship quirks. In "The Wildebeest Implementation", it was revealed through Bernadette that Leonard and Priya plans on visiting India later that summer to meet Priya's parents. In "The Roommate Transmogrification", Leonard finds out that Priya will be moving back to India in the summer while she is in a video chat with her parents. In a moment of anger and frustration, Leonard reveals his relationship with Priya to her parents, thinking that she would break up with him regardless since she will be leaving. This is proven to be false in "The Infestation Hypothesis" when Leonard and Priya are having a dinner date over video chat after Priya goes back to Mumbai.
In the episode "The Good Guy Fluctuation", Leonard is distraught after he kissed Alice, and feels that he has to tell Priya that it is a one-time-thing, which does not mean anything. He confesses to her and she replies that it is fine. Her words confuse Leonard and she tells him that she had sex with her ex-boyfriend. So both strayed. Priya tells Leonard that their cheating actions are not a contest to which Leonard disagreed telling Priya that it is and she won. He then excuses himself and closes his laptop, exclaiming that he does not believe it. It is assumed that Leonard and Priya broke up afterwards, since Leonard describes himself as "single" in the next episode "The Isolation Permutation".
Gallery
