When a leak of information in the African section of British Intelligence is discovered, a security man is brought in to investigate.
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.
A physician is forced to operate on a mystery figure under shady circumstances.
