Robert Vaughn and David McCallum reprised their roles as secret agents Napoleon Solo and Illya Kuryakin respectively.

Directed by John Newland, the film also was released to theaters in the United States in 1966 as a double feature with To Trap a Spy.

agents Solo, Kuryakin, Arsene Coria (Fabrizio Mioni) from Italy, and Namana (Bill Gunn) from Liberia their assignment: they are to take a top secret code to a hidden location.

After each unsuccessfully tries to find out what the other is after, he is gassed into unconsciousness and a THRUSH agent, surgically altered to look and sound like Solo, takes his place.

Solo's latest girlfriend, stewardess Sandy Wister (Sharon Farrell), is miffed that he acts as if he does not know her, but makes no fuss.

When the agents reach their destination, a secret, heavily guarded underground vault in the Swiss Alps (filmed at Griffith Observatory), they are told what is inside.

Project Earth Save is a super weapon, designed by scientists from many nations because possible signs of an alien attack had been detected.