Returning to Earth in the 21st century, the Doctor, Jamie and Zoe immediately find themselves caught in the midst of a crisis. T-Mat - a form of instantaneous transportation essential to the smooth running of life on Earth - is going disastrously wrong. The Doctor discovers that the T-Mat Base on the Moon has been taken over by a group of Ice Warriors, led by the villainous Slaar. Their home a desolate and dying planet, the Martian invaders see Earth as a world ripe for conquest. But before they can colonise Earth they must dramatically after its atmosphere. And so they unleash the Seeds of Death…