Season 1

Leslie Winkle appears at lunch in the Cheesecake Factory. She would like to ask Leonard whether he would like to join a string quartet, since they have previously lost one of their players. Leonard is pleased by the idea and agrees. Naturally, Leonard's friends can't help themselves and start teasing him. When Penny says that Leslie and Leonard would have made a cute couple, Leonard begins to obsess over the idea. After the first rehearsal, which takes place at Leonard and Sheldon's, and once all the players except Leslie and Leonard have left the apartment, Leslie confesses to Leonard that the invitation to the quartet was a pretext and that she is interested in him sexually. So both of them continue practicing what seems like some sort of musical foreplay. After the two of them have played the piece in record time, they disappear into Leonard's room.

When Sheldon comes back home in the course of the evening, he is confused by the tie that hangs on the door of Leonard's room. Since Sheldon has no clue what that sign means, he goes over to Penny so that she may help him with this point. Penny explains to him that the tie on the door to a room is a sign that the couple would like "not to be disturbed". When Sheldon grasps that his roommate is currently having sex, he is visibly displeased with the fact, and does not know how he should behave in this situation. The next morning, after a night on the sofa, Sheldon discovers that someone has been at his whiteboard and that his formulas have been changed. It soon transpires that the changes made solve the equation, which, however, does not please Sheldon, since the correction was made by Leslie.

After some deliberation on Leonard's part, he decides to pursue his relationship with Leslie. When he goes to see her new acquisition in her laboratory and, once there, wishes to extend their intimacy, Leslie tells him point-blank that she has no interest in a relationship, and that the sex was a means to an end, that of satisfying her sexual appetite and releasing endorphins. Moreover, she has no further interest in sexual activity, since she is now satisfied until New Year's Eve.