"The Frost Line Outer Space Defence System is a network of missile bases in Northern Canada, tasked with protecting Earth from extraterrestrial threats. In a transmission to Earth, the Mysterons warn Spectrum that they intend to attack key Frost Line installations. They use their powers to destroy a snowcat driven by maintenance technician Eddie and reconstruct both Eddie and the snowcat as agents in their service. The reconstructed Eddie passes security at two bases – Red Deer and Cariboo – and plants devices inside their ventilation systems that render the air unbreathable, killing 250 personnel at Red Deer and 70 at Cariboo.
In a transmission to Cloudbase, the belligerent Frost Line commander General Ward warns Colonel White that he will fire missiles at Mars if another base is attacked. Fearing the Mysterons' response to such an action, White orders Captain Scarlet and Lieutenant Green to investigate Red Deer. Arriving at the base, the officers discover that the personnel have suffocated not due to the introduction of a toxic gas, but the removal of all oxygen from the air.
Obtaining a Spectrum Pursuit Vehicle from a trapper, Scarlet and Green speed to the next base, Big Bear, only to learn that Eddie has already passed through and is now on his way to the Frost Line command centre. After knocking out a guard who is barring entry to the base, Green removes Eddie's oxygen-depletion device from the ventilation system before it can affect the air. Meanwhile, Scarlet takes the SPV and chases Eddie. The snowcat is carrying liquid oxygen, which Eddie dumps into Scarlet's path to force him off the road. Scarlet draws his gun and fires several rounds into a snow-covered slope nearby; this triggers an avalanche that pushes the snowcat over a cliff, causing it to explode on the rocks below. With the command centre saved, Ward, no less arrogant than before, radios Cloudbase to make peace with White."
This episode was written by the late Shane Rimmer, a Canadian actor and regular to the Gerry Anderson universe, who voiced Scott Tracy in Thunderbirds and, at the time of his death, was the voice of Louie Watterson in "The Amazing World of Gumball" from Season 3 onwards.
The episode also marks a rare moment where Captain Blue sits out the adventure, and Lieutenant Green gets a big role for a change.