When Kate Lethbridge-Stewart responds to a message from ex-UNIT operative Douglas Cavendish, she has no idea what she is getting herself into. Arriving at Cavendish's country retreat, she finds herself caught up in ancient rituals and a demonic power her father had once helped to overcome. Summoned from its resting place by a tyranny from the future, a Dæmon is set to return to Earth. Can Kate and the broken Cavendish save the planet from destruction or future domination?