An alien war machine, the Malus, is affecting the inhabitants of the English country village of Little Hodcombe in 1984 so that a re-enactment of a Civil War battle turns into the real thing. The Malus's aim is to gather sufficient psychic energy to activate itself fully.

The TARDIS travellers arrive to meet Tegan's grandfather - one of the villagers - but become involved in the machinations of Sir George Hutchinson, who is in thrall to the Malus. The battle in 1643 becomes linked through time to 1984 and Will Chandler, a youth from the earlier time, finds himself in the present day. The Malus is defeated when Will pushes Sir George to his death.