Department S

An intruder breaks into a heavily guarded estate in France, yet steals nothing - instead he glues a clown's mask onto the face of a sleeping man, then disappears, unseen. When the man wakes up the next morning and discovers the mask, he dies of a heart attack. Sir Curtis asks Stewart to look into the matter as a personal favor, since the deceased was a friend of his.

So Stewart goes to France and talks to the other people living in the villa, including the beautiful and seductive Nicole (guest star Alexandra Bastedo). His investigations don't go unnoticed, and he soon finds his life threatened by an anonymous voice on a tape recorder demanding that he stop snooping around immediately if he wants to survive he following day. The effect, of course, is the opposite: now knowing that he has a 'real' case on his hands, Stewart declares this an official case for the Department, and soon he, Jason, and Annabelle discover a deadly blackmail scheme - which they have to put an end to, especially since other people's lives have been threatened, too, and the blackmailer has already proved that he doesn't believe in empty threats...