UNIT is called in when an important diplomat is attacked in his own home – by a man who then 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 to 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.
The Doctor and Jo make a test flight in the TARDIS and arrive on the planet Peladon. Seeking shelter, they enter the citadel of the soon-to-be-crowned King Peladon, where the Doctor is mistaken for a human dignitary summoned to act as Chairman of a committee assessing an application by the planet to join the Galactic Federation.
The Doctor and Jo encounter an ancient race of amphibious creatures off the South Coast of England, and soon learn of their plans to seize Earth for themselves – in collaboration with the Master.
When a strange message pod turns up at Unit HQ, the Doctor and his assistant Jo suddenly find themselves involved in another dangerous mission for the Time Lords. The Tardis takes them to Skybase One above the inhospitable Solos. It is the 30th century and the planet is about to gain independence from Earth’s glorious empire. But someone on Solos has other plans and, alarmingly, the natives are slowly mutating into fierce-looking creatures. It’s time for the Doctor and Jo to find out why.
The Master, in the guise of Professor Thascales, has constructed at the Newton Institute in Wootton a device known as TOMTIT – Transmission Of Matter Through Interstitial Time – with which to gain control over Kronos, a creature from outside time.