The Mysterons threaten to destroy London and a truck carrying an atomic device goes missing and is hidden somewhere in the city. The only clue to its whereabouts is the truck driver, found unconscious in the street.

When the Mysterons threaten to destroy key links in the Frost Line Outer Space Defence System, Captain Scarlet and Lieutenant Green race to investigate the mysterious deaths of the personnel at Red Deer Base.

The Mysterons engineer the launch of a Variable Geometry Rocket from Base Concord and their agent, Major Reeves, escapes with the destruct mechanism and target information.

Using the Mysteron pulsator recovered from the complex in Crater 101, Dr Kurnitz has developed a transmission device with which Colonel White attempts to contact the Mysterons and sue for peace.

When the Mysterons threaten to destroy President Roberts within twelve hours, Captain Scarlet is assigned to guard the skeptical President of the United States.