"Avalanche" is the fifth episode of the Supermarionation television series Captain Scarlet and the Mysterons. It was first broadcast in the UK on October 27th, 1967, on ATV Midlands. It was written by Shane Rimmer and directed by Brian Burgess. In this episode, the Mysterons kill and reconstruct a maintenance engineer to cripple the Frost Line Outer Space Defence System.
The Frost Line Outer Space Defence System is a network of missile complexes stretching 3,000 miles. The size and location of the Frost Line Outer Space Defence System are revealed in the initial description in Shane Rimmer's script: "[The Frost Line Outer Space Defence System] stretches in a number of similar bases three thousand miles across Northern Canada." (Bentley, 126.)</ref> across Northern Canada, dedicated to protecting Earth against extraterrestrial threats. When the Mysterons announce their intention to destroy major Frost Line bases, Eddie, a maintenance engineer, is killed by the Mysteron influence, which causes his truck to crash, and reconstructed in their service. The Mysteron agent evades security at both Red Deer and Cariboo Bases and plants devices inside their ventilation systems, rendering the air unbreathable and killing 250 personnel at Red Deer and 70 at Cariboo. On Cloudbase, Colonel White orders Captain Scarlet and Lieutenant Green to investigate Red Deer and uncover the method of attack. General Ward, the bad-tempered Frost Line commander, states that missiles will be launched at Mars if further bases are knocked out: White fears the Mysteron response. At Red Deer, Scarlet and Green discover that the personnel have suffocated not due to the introduction of toxic gas, but the removal of all the oxygen in Red Deer's artificial atmosphere.
Requisitioning a Spectrum Pursuit Vehicle from a local hunter, Scarlet and Green rush to the next base, Big Bear, only to discover that Eddie has already passed through and is now heading towards the Frost Line Command Centre. Knocking out a sentry who bars the Spectrum officers' entry, Green extracts Eddie's depletion device from the ventilation system before it can affect the air. Meanwhile, Scarlet pursues Eddie in the SPV. The truck is carrying a load of liquid oxygen, some of which Eddie empties onto Scarlet's path to force his enemy off the road. Realising that Eddie will deliberately crash his vehicle into the Command Centre to ignite the combustible substance, causing a massive explosion, Scarlet shoots bullets into a snow-lined slope on one side of Eddie. This causes an "avalanche" which throws Eddie's truck over a sheer ridge on the other side. Crashing down the rock face, Eddie is killed when the vehicle explodes and the Command Centre is saved.
Shane Rimmer's script for "Avalanche" starts with a detailed description of the Frost Line Command Centre,which is made up of a number of domed structures: "Deep snow surrounds the plexiglass domes ... From the large central dome acting as a hub, avenues run off in eight directions. At the end of each spoke at varying distances from the centre are smaller domes. Each dome is airtight and serviced by an artificial atmosphere from an air-conditioning dome."
An orchestra of 15 musicians recorded incidental music for "Avalanche" under the direction of Barry Gray on June 11th, 1967 studio session. In the same session, music for another episode, "Spectrum Strikes Back", was recorded.The principal tracks for "Avalanche" are titled "Mountain Pass" (which can be heard as the human Eddie drives his truck through the snow) and "Deadly Mist and Mountain Chase" (which accompanies Scarlet's pursuit of the reconstructed Eddie towards the episode's conclusion).
"Avalanche" marks one of the few occasions in Captain Scarlet and the Mysterons when Lieutenant Green leaves Cloudbase to fulfil a major part in an operation, a fact that is praised by critics Chris Drake and Graeme Bassett. Chris Bentley, in The Complete Gerry Anderson: The Authorised Episode Guide, suggests that some of the detail of Rimmer's opening descriptions was not transferred to the final cut of the episode with complete success.