It starts with Silas regenerating. Ender is there, as is Bazinga, Nylon Tread and Cilan. He becomes a newer version of himself, but is very ill. Silas asks Ender to retrieve his brother Morobus from prison in Ireland, so that his tears can cure Silas.
Ender sets off on this task. It covers around 15 levels.
Once this is done, after fighting through hundreds of Cleavers and Sorcerers, Morobus, naturally, escapes to Skaro. From here he releases an army of Cybermen onto the Earth, hence the name.