Avalanche is the eleventh produced, and the fifth broadcast, episode of the British television series Captain Scarlet and the Mysterons.
The Frost Line Outer Space Defence System is a network of missile complexes stretching 3,000 miles across North America, dedicated to protecting Earth against extraterrestrial threats.
When the Mysterons announce their intention to destroy key Frost Line bases, they kill Eddie - a maintenance engineer - by causing his truck to crash, and then reconstruct him. First targeting the Red Deer and Cariboo Bases, the Mysteron agent gets past both stations' security and places devices inside their ventilation systems, rendering the air unbreathable - killing 250 men at Red Deer, and another 70 at Cariboo...
On Cloudbase, Colonel White orders Captain Scarlet and Lieutenant Green to investigate Red Deer and find out the Mysterons' exact method of attack. General Ward, the belligerent Frost Line commander, states that he'll launch missiles against Mars if more bases are knocked out - and White fears the Mysterons' response, should Ward make good on that threat...
At Red Deer, Scarlet and Green eventually discover that the personnel suffocated not due to poisonous gas, but because all the oxygen in the air has been removed. Having requisitioned an SPV from a nearby hunter's lodge, the two officers rush to the next base, Big Bear - only to discover that the Mysteronised Eddie has already passed through, and is now headed towards the Frost Line Command Centre...
After knocking out the sentry - who's under orders from Ward to prevent Spectrum from entering the base - Green extracts Eddie's oxygen-depletion device from the ventilation system, before it can affect the air. Meanwhile, Scarlet chases Eddie in the SPV. The truck is carrying a large payload of liquid oxygen, some of which Eddie empties onto the path of the oncoming SPV to force Scarlet off the road...
Realising that Eddie may deliberately crash the truck into his target to ignite the combustible substance and obliterate the Command Centre, Scarlet fires several shots from his gun into the snow-lined slope on one side of the track, opposite a sheer drop. This causes an avalanche, which throws the truck over the edge, crashing down the rock face where it finally explodes, killing Eddie and saving the Command Centre.
- The "Fireflash Landing" music suite, made for the Thunderbirds pilot Trapped In The Sky, was reused in this episode.
- The telephone adjacent to the "Might Makes Right" sign on General Ward's desk previously appeared in Thunderbirds Are Go.
- Filming is known to have taken place on 13 April 1967.
- Lieutenant Green’s line “I’m sure glad Colonel White gave me this chance to see some action” seems to refer to the events of Manhunt, when the Colonel refused him permission to join the ground search for Captain Black. It also implies that this episode takes place before Lunarville 7 and Crater 101.