Several murders of nuclear scientists, that baffles Scotland Yard, occur in London around the same time that Bill Locklin, a special officer from the United States State Department, arrives to oversee the transfer of Professor Leon Dushenko, a Russian scientist who has fled the USSR. An attempt on Dushenko's life is made with a monkey's paw-print found at the scene.

The time travellers arrive in Scotland just after the Battle of Culloden. The Second Doctor gains the trust of a small band of fleeing Jacobites by offering to tend to their wounded Laird, Colin McLaren. While Polly and the Laird's daughter, Kirsty, are away fetching water, he and the others are all captured by Redcoat troops commanded by Lieutenant Algernon Ffinch.