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 schemewhich 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...
 

On twentieth-century Earth, the Doctor's old enemy, Lytton, appears to have allied himself with the ruthless Cybermen, who have devised a scheme which, if successful, could completely destroy the web of time and bring the human race to its knees. For when the Cyberplanet 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 tro survive, the Earth must die ...

Eric Saward's novelizarion is based on the original story by Paula Moore, which featured Colin Baker in the role of the Doctor.