A man involved in a car crash is found to have fifty thousand pounds worth of diamonds in his stomach. Steed investigates the man's joke shop, Big Laugh, and then the offices of multi-millionaire Litoff. The burial of one of Litoff's Great Danes is not quite what it seems.

A vastly strong, bullet-proof killer homes in on and destroys several electronics executives. Could it have something to do with Dr. Armstrong's automated work place? Or with the activities of a nearby karate school...

The brother of the late Clement Armstrong sets a deadly trap for Steed and Mrs. Peel, whom he blames for his brother's death at the hands of his murderous creations, the Cybernauts.