"Broadcasting to Earth from Lunarville 7, the Lunar Controller declares the Moon an independent world and a neutral power in humanity's war with the Mysterons. Colonel White sends Captain Scarlet, Captain Blue and Lieutenant Green to the colony to present the World President's written response to this surprise announcement. Scarlet, Blue and Green are also ordered to investigate the Humboldt Sea on the Moon's far side, where orbital surveillance indicates that a new, unauthorised colony is being built.
On arrival at Lunarville 7, the officers are met by the Controller and his assistant, Orson. They are also introduced to the colony's main computer, an artificial intelligence called "Speech Intelligence Decoder" (SID) that identifies humans through recognition discs. After the officers hand over the President's letter, Orson agrees to take them on a trip in a Moonmobile, a vehicle that exploits the Moon's low gravity to jump and glide over its surface. However, when Scarlet suggests a visit to the Humboldt Sea, Orson angrily refuses and returns them to Lunarville 7. While retiring for the night, the officers find that their accommodation has been bugged.
Rising early, Scarlet attempts to request a Moonmobile from SID but discovers that the Controller has re-programmed the computer to accept only his commands. Scarlet also learns that the Controller has declared a state of emergency and ordered the evacuation of Lunarville 7. Scarlet switches recognition discs with the sleeping Controller to trick SID into giving him, Blue and Green a Moonmobile. Travelling to the Humboldt Sea, the officers discover a Mysteron installation under construction in a crater numbered "101".
Returning to Lunarville 7, Scarlet, Blue and Green confront the Controller and Orson, who appear to be Mysteron reconstructions. Scarlet, still wearing the Controller's disc, instructs SID to prepare an Earth-bound shuttle for immediate departure. When SID rejects the Controller's order to lock down the colony, the Controller produces a handgun and repeatedly shoots SID, causing an explosion that kills him and Orson. Scarlet, Blue and Green blast off in the shuttle before further explosions destroy the whole colony."
First episode in a trilogy. Parts 2 and 3 are presented later on.
If the Mysterons are supposedly peace loving aliens as mentioned in the first episode, why would they try to take over the Moon? They must have killed the Lunar Controller and Orson before the arrival of the Spectrum agents.
The Mysterons imply that they share a curiosity about the universe and possibly saw interest in moon colonisation, but supposedly abandoned this when Earth got there first.
But now that the War of Nerves is underway, they likely took control of the Moon colonists with the intent of building the Crater 101 complex to move their planetary forces closer to the Earth. One or two agents, or the hijack of a machine is one thing, but having a base of operations closer to Earth than Mars would give them possibly the power to make the events of "Attack on Cloudbase" a reality.