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.
The episode is one of four that were subsequently re-edited and combined to make Revenge of the Mysterons from Mars (1981), a Captain Scarlet compilation film produced by the New York office of distributor ITC Entertainment.
To learn more about the Mysterons, Spectrum have devised "Operation Sword", the objective of which is to land a spy probe on Mars's moon Phobos in order to capture detailed images of the planet's surface.
[2] After "Shadow of Fear", the regular characters of Captain Grey and Doctor Fawn, who were voiced by Maxwell and Tingwell respectively, continued to appear in the series but only in a non-speaking capacity.
Both Maxwell and Tingwell can be heard in the clip show series finale, "The Inquisition", which includes a flashback to "Big Ben Strikes Again" with the two actors voicing background characters.
[4] Shane M. Dallmann of Video Watchdog magazine calls the miniature model work "excellent" but also notes the violence of the episode, describing the story as "astonishingly downbeat".
Praising Lynn's direction, he writes that the episode "captures Captain Scarlet's dark heart completely, offering a rare instance where the Mysterons succeed against Spectrum.