Shortly after his breakup with Penny, in "The Plimpton Stimulation", Leonard has a one-night stand with Dr. Elizabeth Plimpton (Judy Greer), a physicist he admires, when Sheldon invited her to stay in their apartment for a brief visit. This encounter causes some tension between Leonard and Penny, who is somewhat judgmental of Leonard's doing so. However, the two later resolve their feelings and their friendship continues.