Stranded on Mars, the Doctor and Roz team up with a group of colonists on a journey to find much-needed supplies at the North Pole. But when their expedition is joined by a party of Ice Warrior pilgrims, tensions are stretched to breaking point. Meanwhile, Chris finds himself on Pluto's moon, trapped with a group of desperate scientists in a deadly race against time.

The year is 2157: the Earth has been invaded, and forces are at work on Mars to ensure the mysterious invades are successful. Unless the Doctor can solve the riddle of the GodEngine, the entire course of human history will be changed.