A dizzy delight of double identity escapades. Dudley Jerico (Patrick Macnee) and his partner Wally (Marty Allen) set out to rob corrupt millionaire Victor Rosso (Herbert Lom) of his famous Gemini diamond. Rosso is obsessed with obtaining the stone's twin and Jerico obtains a perfect copy to sell him. Jerico switches diamonds so that he can sell Rosso a 'real' twin. But Mademoiselle Claudine claims she has the twin Gemini diamond. Jerico again switches diamonds and discovers the original is paste. Almost caught, Jerico is rescued by Rosso's secretary, Susan (Connie Stevens) and the two fall in love. Finally Jerico is outwitted and admits defeat but the double dealing is not yet finished.