Tony Britton as Dr. Robert Marriott

Virginia Maskell as Dr. Lucy Byrne

Ian Bannen as Alan Andrews

Kenneth Griffith as Dr Shole

Peter Cushing as Professor Sewell

Thorley Walters as Special Agent Prince

Donald Pleasence as Parsons, alias Bill Brown

Spike Milligan as Arthur, lab orderly

Raymond Huntley as Sir George Gatting the Minister of Defense

Sam Kydd as Slater

Anthony Booth as Parkin

Basil Dignam as Dr Childs

Brian Oulton as Director

Robert Bruce as Levers

John Payne as Iverson

Bruce Wightman as Phil the Barman

Geoffrey Bayldon as Rosson

Andre Charisse as Heller

Ian Wilson as Pin Table Man

Margaret Lacey as Prince's Secretary

Murray Melvin as Teddy Boy