The Antonius family breeds German shepherds. She decides to keep little Rex but a man comes to steal from them on behalf of a rich industrialist, Rudlf Kainz. The dog manages to escape and meets Benny, a little boy unhappy since the death of his father. Alerted by Benny's mother, the Antonius family decides to leave the dog with him. Rex and Benny become great friends. But unfortunately, Kainz runs after Rex and unwittingly Benny discovers a terrible secret. He escapes with Rex but Kainz and his men chase him...