Sheldon's brilliant pen pal friend, renowned quantum cosmologist Dr. Elizabeth Plimpton, is being wooed by Cal Tech. Sheldon invites her to stay with him and Leonard while in Southern California. Sheldon feels protective of her because he feels she is in the same genius class as himself. Leonard is in awe of their temporary roommate. Dr. Plimpton, however, makes it obvious to Leonard that she is interested in him more than just as a fellow scientist. Leonard has a difficult time keeping his intimate encounter with Dr. Plimptom a secret. She doesn't seem to mind as she sets her sights a little more broadly than just Leonard.