 Fearing that his recently-acquired step-mother, Ann Dennis (Margaret Lindsay), is competing with him for his father s affections, and saddened by the death of his dog, young Danny Mitchell (Ted Donaldson), in the first film of the long-running "Rusty" series, seeks consolation in the companionship of a ferocious, Nazi-trained police dog, Rusty (Ace the Wonder Dog), brought to the U.S. by a returning WWII-veteran. The step-mother, with tender understanding, eventually wins Danny over while Danny pacifies his new dog.