The Doctor discovers a starship trapped in a time warp over Earth. Meanwhile, public schoolboy - and secret alien émigré, Turlough, learns he can have that which he most desires, in return for the murder of the Doctor...

The TARDIS lands on Earth six years out of date, stranding the Doctor in 1983 while leaving Tegan and Nyssa to look for him in 1977. Both parties also encounter former companion, Brigadier Lethbridge-Stewart, though curiously, he has no memory of the Doctor at all.

Mawdryn and his eternal mutants reveal the truth: they are exiles, they are in pain and need the Doctor's future 'lives', as afforded by each of his remaining regenerations.

As the Doctor's ethics are challenged and the two Brigadiers finally meet, Turlough plots with the Black Guardian to bring the Time Lord down once and for all...

Deep space, some time in the future. Still following the Black Guardian's orders, Turlough sabotages the TARDIS, forcing an emergency fusion with an apparently deserted starship. But the ship is headed for the notorious plague colony, Terminus. Surrounded by plague victims and space pirates, is the Doctor too preoccupied to notice the greatest threat of all - a threat connected to the position of Terminus at the exact centre of the universe?

The Doctor's party remains divided and scattered as the immense transport docks at the Terminus space station, which turns out to be a leper colony at the exact centre of the known universe. As they find their way around and investigate, Nyssa shows signs of contracting the disease.

Since the Doctor's party represents neither lazars nor handlers, they're presumed to be investigators, which is enough to spark the disgruntled Valgard into challenging Eirak over leadership of the handlers. Meanwhile, as the giant, dog-like Garm takes a terrified Nyssa off for "treatment," Bor returns from the forbidden zone with interesting news about the ship.

In attempting to reopen a doorway into the TARDIS, Turlough activates Terminus' automated fuel-jettisoning sequence. The first time this sequence was engaged, it flung its first of two massive loads of unstable fuel into the distant past, producing the Big Bang that created the universe. This second sequence - if the Doctor can't find a way to shut it down - will release a second massive load, the explosion of which will entirely negate the effects of the first.

After receiving a warning from the White Guardian, the Doctor initially believes the TARDIS has landed aboard an Edwardian clipper ship - but all is not as it seems. While the time travellers are caught up in the omnipotent Eternals' race for the ultimate prize, the Black Guardian's scheme to destroy the Doctor enters the end-game - but which side is Turlough a pawn of?

Captain Striker and his officers reveal themselves to be Eternals, mind-reading creatures who live outside of time and who require Ephemerals (humans and other "time dwellers") to relieve them of their emptiness. They race against other Eternals for the grand prize of Enlightenment, by which to grant their deepest wishes. That can't be good for the universe, but how can the Doctor strategise against beings adept at reading his every thought?

Turlough's panic puts him aboard a pirate ship where Captain Wrack uses his presence to invite all the other captains over for dinner. She's been picking off a few of her greatest rivals in the race and now sees an opportunity to clear the field once and for all and become the clear winner.

Tegan isn't aware she been outfitted with a device to destroy Captain Striker's ship. While the detonation will merely slip the indestructible Eternals back into the realm of eternity, nothing nearly so elegant or as tidy awaits the Ephemerals aboard the ship.