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.

While assessing the security of the castle, Captain Scarlet discovers a secret alcove in the conference hall containing a mounted machine gun manned by Holt.

[7] The illusion of distant lightning bursts was created by flashing an arc welder, while the flames and smoke issuing from the crippled XQR were produced by Jetex fuel pellets attached to the side of the model that was facing away from the camera.

[7] For the crash scene, petrol gel explosives were detonated in the foreground to simulate the plane's destruction; the model was retained so that it could be re-used in later episodes.

[12] The interiors were built by Century 21's art department, who according to Trim were rarely able to replicate the curves of his scale model designs as they made their sets largely out of "flat sheet" materials of limited malleability.

[10][14] Commentators Chris Drake and Graeme Bassett note the mixture of live action and puppetry in the dungeon scenes, which creates forced perspective.

[15] The incidental music was recorded by series composer Barry Gray on 27 August 1967 in a four-hour studio session with a 16-piece orchestra – one of the largest used for any Captain Scarlet episode.

[18][19] "The Trap" is the first episode of Captain Scarlet in distributor ITC's official broadcast order to feature the lyrical version of the end titles theme performed by The Spectrum.

[9] Chris Bentley, author of Captain Scarlet: The Vault, calls "The Trap" a viewer favourite, praising the episode's "atmospheric setting" and "tense, action-packed climax".

[20] Writer Fred McNamara regards "The Trap" as "perhaps the strongest example of how watchable an episode of Captain Scarlet can be if you get the tone right, but almost don't bother with the plot", criticising the story and writing but praising the design and puppet work.

Judging the episode's title "as uninspired as its content", McNamara sums up the premise as "one deafening yawn", arguing that the script's decision to treat the air force leaders as a "soulless mass of a character" rather than as individuals makes the story "unfulfilling".

Though McNamara commends the design of Glen Garry, he believes that the castle setting feels forced, suggesting that Captain Scarlet was merely trying to follow in the footsteps of castle-bound episodes of Stingray and Thunderbirds.