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.