[2] Set in 2068, the series depicts a "war of nerves" between Earth and the Mysterons: a hostile race of Martians with the ability to create functioning copies of destroyed people or objects and use these reconstructions to carry out specific 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 with a reconstruction that later broke free of their control.
The double of Scarlet has powers of self-repair that enable him to recover from injuries that would be fatal to any other person, which make him Spectrum's greatest asset in its fight against the Mysterons.
For the attack sequence, only a small number of spacecraft models were made; flashing light bulbs were placed in the backdrop as a cost-effective way to create the illusion of a larger fleet.
Series composer Barry Gray wrote a full score after Anderson decided that none of the incidental music in Century 21's library suited the episode's grim tone.
[2][3] The previously unseen Cloudbase Radar Room was created using walls and props that were originally built for Thunderbirds Are Go (1966) and earlier Captain Scarlet episodes.
The scenes of Symphony Angel in the desert and the Mysterons attacking Cloudbase presented a number of challenges for Century 21's special effects department, headed by Derek Meddings.
Well, I started to cry, and immediately a voice came down from the recording booth and [producer] Reg Hill, who was directing that particular week, said, 'No, Liz, love.
[1] Series composer Barry Gray devised the score after Anderson decided that nothing in Century 21's musical archive suited the dark nature of the plot.
He praised the script, noting Blue's feelings for Symphony and his conflict with White as examples of Tony Barwick's "humanisation" of the puppet characters.
Describing the episode as highly entertaining even if "shocking and surprisingly bleak", he praises the writing and characterisation – in particular, the death of Rhapsody Angel and the conflict between Blue and White.
"[14] Andrew Thomas of Dreamwatch magazine considers dream sequences an "appalling" narrative technique but describes the episode's scenes of destruction as "very well handled".
He compares it to the Fireball XL5 episode "A Day in the Life of a Space General" (1963), which features an "orgy of destruction with the convenient cop-out of a dream to act as a 'reset button'.
"[16] Chris Drake and Graeme Bassett describe "Attack on Cloudbase" as "tense and exciting", stating that it is "guaranteed to keep the viewer guessing".
This involved the creation of a new ending with computer-animated visual effects and a Mysteron voice-over (supplied by an American actor) implying that the destruction of Cloudbase really happened.