At a null point in space and time on the interface between our universe (n-space) and E-space (exo-space), a slaver cargo vessel becomes trapped. It manoeuvres the timelines using members of the leonine Tharil race as navigators – and they too are the cargo of the grim vessel, held in cryogenic storage in its hold. A Tharil named Biroc flees the craft on the time winds, leaving it moored and trapped, and ventures to the TARDIS, which has also become trapped in a time rift in this strange dimension. The creature manifests itself in the TARDIS console room and imparts to the occupants the warning that he is a shadow of his own past—and of their future; and that the others that are following him should not be trusted. The Tharil then disappears, with the Doctor in pursuit. Romana and Adric are left worried about K-9, whose memory wafers have been shredded by the time winds.
The commander of the slave ship, Rorvik, is increasingly angry about their entrapment in the void and the general apathy of his crew toward their condition. When the ship picks up another object in the void, he and two of his crew members use a portable mass detector to track down the TARDIS. Romana steps outside to confront them and is persuaded to return to their ship with them to examine the damaged warp drive of their ship. Rorvik has, however, worked out she may be a time sensitive like the Tharils and is interested in her for potential profit. Adric and K9 both venture separately into the void in her pursuit.
The Doctor has meanwhile reached a vast stone archway in the void, containing an abandoned banqueting hall, decaying skeletons and partially functioning robot knights known as Gundans. They were built by slaves as part of a revolt against a previous tyranny that ended in a massacre at the feast in the same hall. The slavers used the time winds to descend on worlds and enslave the populations. K9 arrives at the Gateway too and begins to help the Doctor repair the Gundan, which has worn itself out. However, before they can obtain any more information Rorvik arrives with some of his crew and seizes control at the point of a gun. Romana has been taken back to the slaver ship for use as a navigator, while Rorvik has used the portable mass detector to isolate the Gateway. He becomes angry when a Gundan comes to life and walks out of the room straight through a solid mirror, and challenges the Doctor to explain the situation. His response is to walk through the mirror: a step the human slavers behind him cannot take.
Back on the slave ship, a Tharil called Lazlo, who had been left for dead by the slavers when his revivification process seemingly failed, frees Romana from certain death in the navigator’s chair. She hides in the hull of the ship and there finds Adric, and together they work out the slave ship is made of the incredibly dense dwarf star alloy, the remnants of a collapsed star. This heavy metal is the only thing that can hold the Tharils trapped in a single timeline. When they meet K9, the badly damaged robot informs them of dimensional instability and the imminent collapse of the void, which is contracting upon itself. The sheer weight of the slave ship is damaging the delicate balance holding the dimension together. Romana is shortly afterward separated from Adric and K9 and reunited with Lazlo, who takes her through the mirrors too.
Beyond the mirror the Doctor has found Biroc, who explains he was able to pass through because his hand was caught in the time winds. They enter an elegant mansion, seemingly frozen in time, where the Tharil explains that his people were the notorious slavers of the Gateway whose rule was overthrown by the use of the Gundans. As the sorry tale of the decay of their society is retold, the Doctor is reunited with Romana but in a bizarre twist they are returned to the reality of the decaying Gateway in the void once the Tharils’ story has been told – and once more find themselves prisoners of Rorvik.
Based on the reports of the shrinking of the Gateway, the Doctor deduces that the slave ship is indeed the cause of the problems and – more worryingly – there is not much time before the Gateway retracts into nothing. Adric has meanwhile hidden himself inside the MZ, a vast portable cannon which the slavers are going to use on the Gateway and the mirrors in an effort to escape the void. When the weapon reaches the Gateway he uses it to free the Doctor and Romana, and the time travellers flee into the void. When the slavers set out in pursuit they find their ship has moved much closer to the gateway – proof positive that the dimension is shrinking and therefore doomed. In frustration Rorvik fires the MZ at the mirrors, but succeeds only in destroying the weapon. The crazed and infuriated slaver now decides to use his ship’s engines to back-blast through the mirrors and escape the void. It is a foolish move: the backblast from the mirrors engulfs and destroys the ship and all its crew.
The Tharil slaves have, however, been freed by Lazlo and Romana, who has formed an empathy with the race. She elects to voyage through the Gateway with them and help the Tharils. The Doctor gives her K9, who will be restored beyond the Gateway, though he can never return. After they depart, the Doctor uses the knowledge he has gained from the Tharils to successfully pilot the TARDIS through the Gateway and back into N-Space.