"Space Major Reeves, a rocket expert and friend of Colonel White, leaves Cloudbase after completing a tour of the facility. He departs in a motor yacht for the island military installation Base Concord, unaware that Captain Black is watching from the shore. Black uses the Mysterons' powers to induce nausea in Reeves, who falls overboard and drowns in the yacht's slipstream.
A Mysteron reconstruction of Reeves arrives at Base Concord, shoots the control room officer and launches an incendiary variable-geometry rocket (VGR) under the control code "ZERO". He then escapes in a J-17 fighter carrying the flight program unit, leaving the base personnel no way of knowing the VGR's target or which of the 10,000 listed codes must be transmitted to trigger its self-destruct. In addition, for reasons unknown the VGR is not appearing on radar.
The commander alerts Spectrum and White dispatches Captains Scarlet and Blue to Base Concord. Meanwhile, the Angel squadron are launched to track down Reeves and recover the unit. Reeves is quickly intercepted but refuses to surrender, instead crippling Melody's aircraft with the J-17's machine gun and forcing her to eject before she crashes into the ocean.
At Base Concord, Scarlet, Blue and the base personnel realise that the VGR would be invisible to radar only if it were travelling upwards, and that because its descent would be equally vertical the only plausible target is the base itself. A replacement unit is installed and the personnel use it to transmit codes one after the other in a desperate effort to find the one used by Reeves. In the air, Reeves ignores Rhapsody's order to surrender and deliberately crashes his fighter, causing the original unit to fall to the ocean floor.
Minutes before impact, and with Base Concord fully evacuated except for Scarlet and Blue, White radios his officers and instructs them to leave. However, in a last-ditch effort to save the base, Scarlet and Blue disobey White and carry on trying codes. "AMEN" is rejected, but at the same time the sunken unit is knocked over by an underwater current and the impact causes it to re-transmit "ZERO", triggering the VGR's self-destruct.
Scarlet and Blue return to Cloudbase believing that they miraculously found the correct code. When the truth is revealed, White reprimands the officers for their insubordination but stops short of court-martialling them, recognising the value of their bravery in the fight against the Mysterons."