"Lunarville Seven," stylised as "Lunarville 7," is the twelfth instalment of Captain Scarlet And The Mysterons and the first of the three instalments that comprise "The Lunarville Trilogy." In it, Captain Scarlet, Captain Blue, and Lieutenant Green are dispatched to Earth's Moon to investigate unauthorised construction taking place in the Humboldt Sea.
The Controller of Lunarville Seven declares Earth's Moon officially neutral in the Mysterons's war of nerves against Earth. Meanwhile, intelligence from Spectrum's orbital reconnaissance has disclosed new, unidentified, and unauthorised construction taking place in the Humboldt Sea, on the far side of that moon. Colonel White, understandably suspicious, dispatches Captain Scarlet, Captain Blue, and Lieutenant Green. Ostensibly, they are to deliver a letter of response to the Controller that the World President has written. But secretly, they are to follow up on Spectrum's orbital reconnaissance and investigate the base.
When the Spectrum agents reach Lunarville, the Controller meets them, as does his assistant Orson; they are shown Lunarville 7's control computer, "SID," or Speech Intelligence Decoder, which uses recognition discs to identify humans. After the agents hand over the World President's letter, Orson agrees to take them on a trip outdoors in a "Moonmobile." This vehicle relies on the low gravitation of Luna (the technical catalogue name for Earth's Moon) to jump and glide over the lunar surface. But Orson refuses to go as far as the Humboldt Sea. When the Spectrum agents uncover that their accommodations have had listening devices concealed within them, their suspicions of their hosts mounts.
Captain Scarlet wakes in the early hours of the morning to find himself unable to request a Moonmobile from SID; the Controller has reprogrammed SID to reject all other commands than his own. The Controller has also declared a state of emergency and ordered all personnel to evacuate Lunarville 7. To trick SID into granting the Spectrum agents Moonmobile access to the Humboldt Sea, Captain Scarlet secretly exchanges his and the sleeping Controller's recognition discs. In the course of the said Moonmobile access, Captain Scarlet, Captain Blue, and Lieutenant Green, to their shock, discover that a Mysteron computer complex is under construction in an unnamed lunar crater marked as "101" on the map.
Captain Scarlet, Captain Blue, and Lieutenant Green return to Lunarville 7 to confront the Controller and Orson, both of whom they suspect of now being Mysteron likenesses of the originals. On Captain Scarlet's orders, SID prepares an Earth-bound shuttle for immediate departure--and the Controller, losing his composure when SID rejects his commands, draws a pistol and fires on SID, triggering an explosion in which he and Orson are destroyed. Captain Scarlet, Captain Blue, and Lieutenant Green lift off, in the shuttle, from Lunarville 7 before additional explosions completely destroy it.
The reports from the Spectrum agents dress the stage for the mission whose focus "Crater One Hundred And One" is.
- The speech with which the Controller of Lunarville 7 begins the instalment states that approximately 4,000 humans consider Luna home. Dialogue edited out of Tony Barwick's original script was intended to discuss the self-sufficiency of lunar bases in more detail; one example of the omitted dialogue states, "We were all born on Earth, but I see a future where men will be born, spend their lives, and die on the Moon."
- The Moonmobiles as shown in "Lunarville Seven" and "Crater One Hundred And One" influenced Derek Meddings's designs for the SHADO Moonmobiles in UFO, the later Century 21 TV programme that was the first Century 21 Television Production to be produced entirely in live action.
- The incidental music in this instalment makes extensive use of an Ondes Martenot, which instrument series composer Barry Gray himself played, for the sequences situated in and around Lunarville 7. The music for this instalment and that for "The Launching" were both recorded in the same studio session, on 23 July, 1967. Four instrumentalists were in attendance at that session.