By the late 21st century, mankind has become totally dependent on T-Mat to transport people, food and medicines around the world. When the system breaks down, Earth is soon crippled by global shortages. Traveling to the relay station on the Moon, the Doctor and his companions discover the horrific truth: the Ice Warriors have hijacked T-Mat, and intend to claim the planet for themselves. Can the Doctor defeat the Martians before Earth is smothered in deadly fungus, and the human race is condemned to extinction?