The Mysterons are usually thought of as an invisible enemy based on Mars but the reality is that there are no Mysterons on Mars: Spectrum is only fighting the computers they left behind centuries earlier.
Between pre-production and filming, the approach to voice casting changed. The original intention was that each episode would feature a "guest star" character voiced by a well-known actor. In the case of this first episode, the World President was to have been voiced by an artist "of the Patrick McGoohan calibre" Although the puppet was modelled on McGoohan, the actor was unable to commit to the role, and the "guest star" idea was later abandoned due to budget constraints.
The pilot script, written in August 1966, contained several key differences from the finished episode:
- It elaborated on Captain Scarlet's new Mysteron nature, stating that the reconstruction is a "mechanical man" rather than true flesh and blood.
- There was a longer Cloudbase introduction, in which the Angels are launched but subsequent dialogue reveals that this is a drill (in the finished episode, only the launch is shown)
- An additional scene in which Symphony and Melody Angels watch from the Cloudbase promenade deck as the Spectrum jet takes off.
- A Scene of Captain Blue converting the SPV's removable power unit into a personal rotorcraft, or "minicopter", to take on Scarlet (in the finished episode, he wears a jet pack instead).
- An additional line of dialogue stating that the original Helicopter A42 crashed half an hour before its reconstruction attacked Captain Blue.
- The script also indicated that the defeated Scarlet would be revived by Spectrum using a "specially-designed computer", whereas in the completed episode he returns to life of his own accord.
A stunt puppet with slightly different facial features took over during the filming of Captain Brown's detonation to prevent the original from being damaged. This puppet was also used in promotional photography for the series.
This installment was also the only one to have this introduction, which was given by George Victor "Edward" Bishop, who provided the character voice of Captain Blue: "The finger is on the trigger... about to unleash a force with terrible powers beyond the comprehension of man. This force we shall know as the Mysterons. This man will be our hero, for fate will make him indestructible. His name: Captain Scarlet."