UNIT is called in when an important diplomat is attacked in his own home - by a man who vanishes into thin air! The Doctor and Jo spend a night in the 'haunted' house and meet the attackers, who have 'time-jumped' back from the 22nd century in the hope of changing history. Travelling forward in time, the Doctor and Jo find themselves trapped in a future world where humans are slaves and the Daleks have already invaded. Using their ape-like servants, the Ogrons, to maintain order, the Daleks are now the masters of Earth. As the Doctor desperately works to discover what has happened to put history off-track, the Daleks plan a time-jump attack on the 20th century.