Dudley Jerico (Patrick Macnee) sets out to rob corrupt millionaire Victor Rosso (Herbert Lom), of his legendary Gemini diamond. Rosso is obsessed with obtaining the stone’s twin so Jerico obtains a perfect copy of the stone as part of his plan to swindle Rosso. But before he can fleece him a mysterious woman from Paris called Mademoiselle Claudine (Connie Stevens) appears, claiming she has the twin Gemini diamond. Rosso knows that one of them is lying, but which one..?