While out on a late-night stroll Marty Hopkirk witnesses the murder of a Mr. Howarth by the hitman, Rawlings as Howarth enters his home.
He travels back to Jeff's apartment to wait for him, whereupon Jeff turns up in the early hours of the morning with a beautiful girl in tow. Marty demands Jeff send his date home and call the police, which he eventually begrudgingly does, travelling with Inspector Clayton to the house where the murder took place. However, when they arrive they find his wife and it appears that Mr. Howarth is back alive and well.
Jeff leaves after Howarth threatens to sue him, only to arrive back the next morning to apologise to Mrs. Howarth, again at Marty's insistence. Marty sees Rawlings in the study and calls Jeff in, by which time Rawlings has disappeared into the secret room branching off it. Engrossed with the case, Marty spends time in the Howarths' house, whereby he learns that both Mr and Mrs Howarth have been murdered and replaced by impostors. Insisting Jeff returns after he locates the bodies in the basement, Jeff returns to the house again, only to find the bodies are no longer there.