Raj's sister, Priya (Aarti Mankad), was in town for one night on a layover. The back story of this episode involved a five year old agreement that Howard and Leonard made where they promised that nothing would happen between either one of them and Priya. By the evening, Priya had appeared at Leonard and Sheldon's door, while Sheldon was busy with a text based video game. This started the dilemma of Leonard having to sneak Priya past an unsuspecting Sheldon. Though they were successful that evening, they however, could not get past Sheldon and his algorithmic thinking in the morning, forcing Leonard to come clean.
Although Sheldon was at first upset at what happened the night before, he still offered to provide an alibi for Leonard to fall back on. By the time that Sheldon, Leonard, Howard, and Raj were all sitting together in the cafeteria, a discussion on Caprica arose, which put Leonard in a situation where he could be dishonest about the night before. However, it did not take Leonard much time to choose to be honest and open about what had happened the night before. The confession triggered further revelations that turned things between the four ugly in just a blink of an eye, but they were reminded of the importance of their friendship to overlook each other's past misconducts.