When Leslie Winkle asks Leonard to join the faculty string quartet, he has no idea that it may lead to more personal endeavors, especially as their first and only experience in dating each other went nowhere. Leonard's not sure how to reconcile his feelings for immediate sex with Leslie versus his want to date Penny, especially if Penny finds out that he is in a potential relationship. Sheldon's not sure what the protocol is when a roommate has a spontaneous "guest". But Sheldon is aware that by the end of Leonard and Leslie's encounter, he feels violated. And Leonard's feelings may change when he understands Leslie's motivations.