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.
When the Mysteron Detector reveals his true nature, Indigo's double sabotages the conference by activating the controls that lower the hunting lodge into the ground.
The lift becomes inoperable when the lodge starts its descent, removing the other delegates' sole means of escape and forcing Scarlet to pursue Indigo in a second car to retrieve the key.
He questions the game reserve's apparent lack of security as well as the role of Indigo, noting that despite being a Spectrum captain he has been given the "humiliating task" of waiting on the conference delegates.
[10] Writer Fred McNamara praises "Spectrum Strikes Back" for "paying heed" to past Captain Scarlet episodes, stating that this continuity "highlights the narrative ambition that [the series] sometimes displays".
He is less complimentary of other aspects, calling the plot "convoluted" and the conclusion "unfulfilling" and generally commenting that the episode "[relies] more on worldbuilding and character dynamics than explosion" (though adding that the former help to make the series "feel more real").
[11] In a review for Andersonic, Vincent Law names the episode "one of the most convoluted and inconsistent" of Captain Scarlet, criticising elements such as the design of the hunting lodge: "... descending underground for reasons of secrecy is bizarre, as surely any passer-by would become immediately suspicious when a non-descript building started sinking into the ground!"