This episode concludes from where Captain Scarlet, Captain Blue and Lieutenant Green left the remains of the Lunarville 7 compound, where before they discovered a brand new Mysteron complex being assembled in Crater Number 101 of the Humboldt Sea, situated on the far side of the Moon. The power source will also provide Spectrum with ample opportunity to somehow come into direct contact with The Mysterons themselves, to try and come to a peaceful settlement.
All three men volunteer to carry out a full-scale expedition by infiltrating the complex and removing its power source, so it won't become of any harmless use and so the Mysterons will not be able to reconstruct the whole complex after it's destruction.
However, the bomb which is transported to destroy the entire complex is set for 10:00 S.E.T. instead of the agreed Midnight deadline, by a Mysteron agent, disguising himself as a technician officer of Lunarville 4, the space-base that equipped and informed the 3 Spectrum officers, in order to perform the mission.