A Variable Geometry Rocket is hi-jacked by a Space Major Reeves, an old accomplice of Colonel White's, who launches the space-plane and makes a quick departure in a fighter jet with the rocket's flight programme unit, which only he knows the 4-letter self-destruct code, to obliterate the rocket before it knocks out an essential target, which happens to be Base Concord, the state-of-the-art airbase where the rocket originally left from.
Captains Scarlet and Blue are assigned to self-destruct the rocket, before it reaches Base Concord, but to accomplish this, they have to check 10,000 possible 4-lettered code-words, starting from 'ABLE', and they don't even know that Reeves has set the destruct code-word as 'ZERO'.
