A low-ranking Secret Service agent is conned into supplying information to Eastern Bloc countries. Although he is not a suspect due to his unimportant position, when his office partner is hauled in as a suspect he realises he has got himself into very deep water.
The Doctor, Jamie and Zoe return to Earth and become embroiled in the schemes of Tobias Vaughn. They meet up with an old friend, Colonel (now Brigadier) Lethbridge-Stewart, and some old enemies, the Cybermen.