The Frost Line Outer Space Defence System is endangered by the Mysterons when they destroy a maintenance truck, murdering its engineer/driver in the process, as a cover to infiltrate the defence bases sited in Northern Canada, where the Mysteronised engineer intends to proceed in removing all oxygen from the bases' self-contained atmosphere.
With bases' "Red Deer" and "Cariboo" already taken down, the next base in the ring, "Big Bear", is the Mysterons' final target. But if the base is attacked also, the commander-in-chief of the defence network plans to commence a vengeful missile strike on Mars, where the main Mysteron complexes are established.
Colonel White desperately wants to prevent this from happening, and so, he assigns Lieutenant Green to his first off-base, "in-the-field" assignment, along with Captain Scarlet, to evaluate just how the bases are being cut off, and stop the maintenance engineer from triggering off a global disaster.