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Raj's sister, Priya, a corporate lawyer, has a 24 hour layover in Los Angeles on her way to Toronto on a work trip. When the guys met her the first and only time five years ago when she last was in Los Angeles, Howard and Leonard made a "pinky swear" pact not to hit on her, much to Raj's pleasure as he wanted and still wants to protect his little, innocent sister. Priya ends up not being so innocent as she wants to have a casual fling with Leonard on this trip, he who complies and would actually want it to be more than just casual. When Sheldon catches Priya in their apartment when she shouldn't be there, Sheldon devises an elaborate but totally implausible alibi for Leonard in the probability - at least in Sheldon's mind - that Raj and or Howard will ask. This incident brings up the issue of betrayal amongst the four friends.