Handing in his helmet at the end of a short-lived career as a police constable after he mistakenly tries to arrest the Chief Constable, Harry Bell comes to the rescue of a stray dog on his way back to his digs, who licks his fingers and says "thanks".

Recovering from the shock of hearing a dog speak, he gets back to find that his landlady is heavily in debt, according to her accountant. Still reeling from the magical story the dog tells him, he sets out to prove his landlady's innocence, spurred on by his devotion to her daughter, Sheila.

With the help of his old desk sergeant, Harry joins a suspected PR company to investigate their accounts, aided by an undercover female police officer, whose amorous advances dash his romantic hopes when Sheila jumps to conclusions.

Spurned by Sheila and heavily involved in a murder and drugs enquiry, Harry is determined to solve the case, help his canine friend, and win back his love, despite the best efforts of an inept and officious inspector.