Danny Mitchell (Ted Donaldson), a young boy in the American town of Lawtonville, is grieving over the loss of his dog. He is also struggling to adjust to his new stepmother, Ann (Margaret Lindsay), and has a difficult relationship with his father (Conrad Nagel) - causing him to call on Dr. Banning, a psychiatrist (Addison Richards) for assistance. However, Danny befriends Rusty, a ferocious German shepherd who was brought to the United States from Germany during World War II. Having worked a police dog for the Gestapo, however, Rusty is ill-tempered and Danny struggles to train him.
A subplot involves two Nazi saboteurs (Arno Frey and Eddie Parker) who arrive in Lawtonville, attempting to evade the Coast Guard and blow up an installation. They ultimately try to take Rusty by speaking to him in German.
The dog portraying Rusty would change over the course of the series, with Flame having the most appearances as the character. Danny's parents would also be portrayed by different actors in almost every installment.