S4, E3: The Zazzy Substitution
The guys are concerned when Sheldon searches for an alternative to human companionship after he and Amy break up. Can they help restore his faith in people? Meanwhile, when things take a turn for the worse and Sheldon starts bulk buying pet cats, Leonard gets Sheldon's mum to pay him a visit.
Starring Laurie Metcalf, Mayim Bialik