The series was the first made by the Andersons after the success of Thunderbirds.
The basic premise, played out over 32 episodes, was that a special group, Spectrum, defend the Earth from the insidious plans of the alien Mysterons.
On a mission to Mars in 2068 a Mysteron 'city' was destroyed by Captain Black leading the Mysterons to declare war on Earth. The Mysterons have the ability to replicate any person or device (retro-metabolism) and use this skill to conduct a war of terror against Earth - primarily aimed at the world leaders, major cities, industrial and defence establishments and, of course, Spectrum and its airborne Cloudbase headquarters. The Mysterons are never seen, their presence is indicated by two circles of light tracking across the scene, their actions on Earth are always through their replicated intermediaries, often Captain Black, who the mysterons killed and then revived as their first agent.
Spectrum personnel have military ranks and colour based code names, hence Captain Scarlet. They were headed by Colonel White and other characters included Captains Blue, Brown, and Magenta, Lieutenant Green, and the five female fighter pilots, who have a different collective codename - the Angels and are individually Destiny, Symphony, Melody, Rhapsody and Harmony.
The Captain Scarlet character is special in that he was killed and made a Mysteron agent in the first episode. Their hold on him is broken when he falls 250m from a tower and survives. It is found that their regeneration process has made Scarlet indestructible, a advantage he often needs in subsequent episodes.
As in the Andersons' previous puppet series, the characters' eyes and mouths were operated electronically, but in Captain Scarlet the control mechanism was placed in the puppets' chests rather than their heads. This meant that the puppets no longer needed oversized heads to accommodate the mechanisms and could be built with normal proportions for the first time. In order to enhance the sense of realism further, the puppets were never seen walking, as it was impossible to make their legs move realistically. For this reason characters are often seen standing on moving walkways or even sitting at moving desks, and there are of course any number of futuristic land, sea, air and space vehicles for them to ride in, courtesy of special effects director Derek Meddings and his miniatures unit.