Set in 2068, the series depicts a "war of nerves" between Earth and the Mysterons: a race of Martians with the power to create functioning copies of destroyed people or objects and use them to carry out acts of aggression against humanity.
Earth is defended by a military organisation called Spectrum, whose top agent, Captain Scarlet, was killed by the Mysterons and replaced by a reconstruction that subsequently broke free of their control.
Following a number of unexplained Spectrum hovercraft crashes in the Australian Outback, Colonel White dispatches Captains Scarlet and Blue to the Koala Base training facility, where he suspects that a double agent is sabotaging the fleet.
All four men – including Scarlet, who successfully removes the hovercraft's control panel – jump to safety before the vehicle crashes into a rock formation and explodes.
[1] To remedy this, Barwick wrote a flashback to the first episode into each script to reduce the amount of new footage that would need to be shot; this would allow "Dangerous Rendezvous" and "Traitor" to be filmed in a combined 15 days instead of 20.
[1] Due to this recycling of old material, Barwick's script for "Traitor" specified that no footage from the first episode was to appear in the clip show series finale, "The Inquisition" (which instead uses scenes from "Big Ben Strikes Again", "Crater 101" and "The Trap" for its flashbacks).
The original script makes clear that there is no traitor (and the Mysterons' only involvement has been to cause unrest within Spectrum), that the valve failed due to the extremely high temperatures in the Outback, and that the fire in Scarlet and Blue's quarters was an accident.
"[7] Commentators Chris Drake and Graeme Bassett praise the design of the hovercraft, describing it as "an impressive addition to the Spectrum fleet" (though its exact function within the organisation is never explained).