Donald Douglas

Transported back to the Earth of the distant future from the Ark, the Doctor, Harry, and Sarah discover that they have unwelcome company. An alien warrior is conducting deadly experiments to assess the suitability of the human race as an adversary prior to a full scale invasion of Earth.

Transported back to the Earth of the distant future from the Ark, the Doctor, Harry, and Sarah discover that they have unwelcome company. An alien warrior is conducting deadly experiments to assess the suitability of the human race as an adversary prior to a full scale invasion of Earth.

A double-agent, codenamed Merlin, steals a new sleep gas and tries it out on Steed. Awaking 24 hours later, with Merlin his prisoner, Steed can't turn him in… as if everyone has vanished.