Doctor Who and the Time Warrior — Terrance Dicks

His spaceship crippled in an interstellar battle, the Sontaran warrior Linx, is forced to crash-land on Earth. He arrives in the Middle Ages, a time too primitive to provide the technology he needs to repair his ship. Allaying himself with the local robber chief, Linx uses his powers to "borrow" scientists and equipment from twentieth-century Earth.

Doctor Who tracks down the missing scientists and journeys into the past to save them. But can he defeat the ruthless Linx and his savage human allies before the course of human history is changed forever?

Doctor Who and the Time Warrior is based on a 1973 TV serial starring John Pertwee.