In the late 21st century, the human race has become totally dependent on T-Mat, a revolutionary form of instant travel. The Doctor, Jamie and Zoe arrive on Earth just as T-Mat is suffering a malfunction. Sinister Ice Warriors from Mars have seized the lunar T-Mat station to launch an invasion of Earth. Their preliminary plan is to transport deadly seedpods that will unleash a fungus capable of absorbing the world's oxygen. The Doctor must race against time to defeat the invaders - or the human race will be doomed to suffocation.