Set in 2068, the series depicts a "war of nerves" between Earth and the Mysterons: a hostile 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 murdered by the Mysterons and replaced by a reconstruction that later broke free of their control.
In "Model Spy", after the Mysterons threaten to assassinate a French intelligence agent, Captains Scarlet and Blue and Destiny and Symphony Angels go undercover to protect the target.
Believing that Verdain's death would be disastrous for the Service, Colonel White predicts that the Mysterons will make an attempt on the spymaster's life at an upcoming fashion show in Monte Carlo, and assigns a team of four – Captains Scarlet and Blue and Destiny and Symphony Angels – to protect him.
The agents go undercover as employees of Verdain, with Scarlet and Blue (going by their real names Paul Metcalfe and Adam Svenson) posing as a public relations officer and a photographer, and Destiny and Symphony as fashion models.
Remembering that the suspended monotrain from the Thunderbirds episode "Brink of Disaster" had been difficult to film, Trim designed the new train to run on a OO gauge model railway track, which proved much easier to shoot.
[3] The model shots of Verdain's yacht feature a background painting of Monte Carlo Bay, which was also produced by Trim and had previously appeared in the Thunderbirds episodes "The Man from MI.5" and "The Duchess Assignment".
He compliments the French Riviera setting and "impressive" monotrain crash, as well as the originality in casting fashion models as Mysteron agents and having the Spectrum team abandon their codenames during their assignment.
")[10] Shane M. Dallmann of Video Watchdog magazine writes that although the "gimmicky" episode "has its silly moments", it is significant for revealing Scarlet and Blue's real names as well as for the "brutal highpoint" of Verdain being pushed out of Black and Helga's moving car.