The Mysterons's plan is to ground Earth's new World Space Fleet. They mean to do this by attacking Hotspot Tower, in the Arctic Circle, location of the only known mine for "tritonium," hardest of known metals. (A new fuel formulation for the World Space Fleet's spacecraft will propel them to speeds so high that their nose cones would overheat to, nay past, the melting point if constructed of any metal other than tritonium.) To this end, they have already taken over Nielsen, the regular maintenance technician, by entrapping him outdoors and causing his death of exposure.
The mine relies on electrical heating elements to melt the ice-caps, and the Mysterons direct the Nielsen likeness to disconnect their high-voltage power source, which will crush the mine and render it useless. Captain Scarlet and Captain Blue are dispatched to Hotspot Tower; to stop the Nielsen likeness, Captain Scarlet first swims to the mine wearing a heated suit. After he has electrocuted, and thereby destroyed, the Nielsen likeness, using a bridge between the mine and its control station that is already falling victim to the ice, Captain Scarlet does manage to make his escape.