When his colleague is shot dead by a burglar during a chase in North London, PC Frank Mason vows to bring the killer to justice. After volunteering to become a dog-handler he finds he soon he has a new partner to help him on his mission. Rex, the seemingly untamable Alsatian, quickly shows an aptitude for police work, and Frank moves the loyal canine into his home. And soon Rex starts to take over his life in more than Frank anticipated, much to the irritation of his girl-friend Pat, who begins to resent the growing bond between the two. In the meantime, the killer has been lying low. Until, strapped for cash, he breaks into a factory. The 999 call brings Frank and Rex running in hot pursuit... and the situation swiftly turns violent.