The Plimpton Stimulation

Summary
Dr. Elizabeth Plimpton (Judy Greer), a cosmological physicist from Princeton University, comes to Cal Tech for a visit. Sheldon arranges her trip, and offers her to stay in his bed (with Sheldon sleeping on the sofa), rather than a hotel which he perceives as less hospitable. Leonard tries to impress her by talking to her about her work, but she's interested in having sex with Leonard, which he very much welcomes. The next morning, Sheldon is completely oblivious to what happened overnight, but Penny immediately detects what happened based on Leonard's and Elizabeth's body language, and becomes jealous, because she broke up with Leonard not too long ago. Later that day, at the Cal Tech cafeteria, Dr. Plimpton meets Raj and wants to bed him, too. That night, Howard and Leonard come to Raj's apartment for Halo night and find Elizabeth there, so she suggests sex with all 3 guys, but Raj manages to lock the other 2 out of the apartment. At the end of the show, Leonard and Penny discuss whether or not he needs to explain to Penny what happened. Although Penny keeps insisting that he doesn't need to explain anything, Leonard explains that he had sex with Dr. Plimpton because "she let him".

Title reference: Dr. Elizabeth Plimpton comes to visit Sheldon for the purpose of science collaboration but winds up seducing the three other guys and having sex with Leonard and Raj.