A diamond raid in modern-day London...a secret base hidden deep in the heart of the city's sewer system...a cold and desolate planet light years from Earth...and a daring plan to alter the entire course of interplanetary history...

On twentieth-century Earth it appears that the Doctor's old enemy, Lytton, has allied himself with the ruthless Cybermen. The Cybermen have devised a scheme which, if successful, could completely destroy the web of time and bring the human race to its knees.

When the Cyber-planet of Mondas was destroyed in 1986 the Cybermen were forced to retreat to the planet Telos. Now they have journeyed back in time to prevent the destruction of their home world. And for Mondas to survive, the Earth must die...