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.
After Leonard got into a spat with his room mate, Sheldon, he retreated to Penny's apartment. While they are there, they get into a dialogue that recounted how Leonard and Sheldon first met, leading to multiple flashbacks.
There is a certain type of woman that can go into an established close-knit group of men, insert her sexuality within, and destabilize everything. In this episode of The Big Bang Theory, that woman is Dr. Elizabeth Plimpton. Unfamiliar with the opposite sex, Sheldon used his limited knowledge to prepare for her arrival and stay at his and Leonard's apartment.
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