The Doctor, Tegan and Turlough arrive in Little Hodcombe, where the townspeople's re-enactments of English Civil War battles are causing a dormant entity, the Malus, to re-awaken.
When Wayne has to go and live with his grandfather Albert, they both initially resent the arrangement. Hostilities soften when they discover some common interests.
Two teenage boys try to help a tiny spherical alien get back to its mothership, while the army and a devious petty crook pursue the creature for its wonderful powers.
