While out grocery shopping, meek, middle-aged Samuel Smith and family pet Rover are run over by a speeding car. Fortunately for them, an ambulance shows up right away. Unfortunately for them, the ambulance attendant mistakenly treats Sam with dog plasma and Rover with human plasma. Both immediately recover - after which Sam starts erupting into bouts of dog-like behavior and Rover begins walking and talking like a human being, much to the consternation of the people around them, especially Sam's wife, Margaret.