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.
Scarlet's double has a self-healing power that enables him to recover from injuries that would be fatal to anyone else, making him Spectrum's best asset in its fight against the Mysterons.
After meeting with Colonel White regarding a "special assignment" concerning the Mysterons' threat, Captain Scarlet visits a casino and, with uncharacteristic poor judgement, ends up losing $5,000 on roulette.
Their aim is to detonate a nuclear device within the city, creating a chain reaction powerful enough to obliterate the North American continent.
From a map left at the ranch house, Blue learns of the threat to Nuclear City and relays this information to White.
[1] The casino set incorporated props that had previously appeared as part of the Creighton-Ward Mansion dining room in Thunderbirds.
[6] Andrew Pixley and Julie Rogers of Starburst magazine consider Black's murder of Mason, accomplished by running the attendant over with his car, as one of the series' more violent moments, also noting that Black kills a filling station worker in "Manhunt" by crushing him with a car lifter.
[7] Writer Fred McNamara criticises the episode's "clumsy, scrambling premise" and "bizarre" structure, commenting: "So much of it is stitched together so loosely.
Despite this, McNamara also considers the episode to be "full of visual strengths" and engaging "human aspects", praising the "harrowing sight" of the SPV being targeted by the Angel squadron as well as Ed Bishop and Francis Matthews's "crisp, stirring" voice acting during the scenes of the concerned Captain Blue's search for his friend Scarlet.