Season 4, Episode 10 - The Alien Parasite Hypothesis
Aired December 9, 2010
Amy is confused by an unusual sensation she felt in the presence of Penny's ex, Zack. With Sheldon's help, Amy realizes she was experiencing feelings of sexual arousal, so Sheldon sets out to help satisfy her needs. Meanwhile, Howard and Raj battle to find out who is the superhero and who is the sidekick in their friendship.