Inside the TARDIS, the Doctor and his companions, exciting from E-Space into N-space (our space), are trapped in Null Space, a white void between universes. A slave vessel run by Captain Rorvik (Clifford Rose) is also trapped. Lion-like slaves called Tharils are used as navigators because they have an intuitive understanding of space-time. The current navigator Biroc (Davis Weston) escapes and crosses to the TARDIS on the Winds of Time. He warns of Rorvik’s treachery. K9’s memory wafers are shredded by the Winds of Time, destroying his long-term memory. The Doctor leaves to explore on his own and discovers robots called Gundan—creepy creatures that look like skeletons in suits of armor. They have memory wafers like K9.
Rorvik and his crew have discovered the TARDIS. Romana leaves to talk to them. Rorvik dupes her into returning to their ship to look at the engines, but they enslave her like a Tharil because they believe she has a sense of time. When Romana does not return, Adric and K9 go to look for her but are separated. Adric hides aboard Rorvik’s ship, K9 finds the Doctor. They repair a broken Gundan (Robert Vowles) who tells him the Gundans were built by slaves to overthrow their masters. Rorvik arrives with his men and see a Gundan walk through a mirror. Rorvik reveals that he has Romana captive, but the Doctor simply walks through the mirror himself.
Aboard the slaver ship, Romana is greeted by a Tharil named Lazlo (Jeremy Gittens). She hides in the hull and finds Adric. They figure out that the ship is made from heavy dwarf star alloy. K9 shows up and warns that Null Space is becoming unstable because of the ship’s weight. Romana rejoins Lazlo and passes through the mirror, but Adric remains behind.
On the other side of the mirror, the Doctor and Romana are reunited with Biroc in a stable universe. Biroc explains that the Tharils themselves were slave masters, travelling on the Winds of Time, ravaging planets and enslaving species until the Gundan revolt. The Doctor and Romana return to Null Space and are captured by Rorvik. Null Space is shrinking. Rorvik’s crew try to blast through the mirrors, but the mirrors are indestructible. The gateway and the ship are now within sight of each other. Romana regroups with Lazlo and Adric, and they free the remaining Tharils and head for the TARDIS. Rorvik tries once more to break the mirror, but the blast is reflected into the ship, destroying it and killing all. But the TARDIS is freed.
Romana announces that she does not want to return to Gallifrey and will be staying with the Tharils to help them put right their past crimes. The Doctor gives her K9 because passing through the mirror will restore his memory. In turn, the Tharils tell the Doctor how to escape the void back into our space.
The story was re-written extensively. A lot of comedy was dropped. This was not only the last Story for Lalla Ward, who wanted out, but Tom Baker announced that he was leaving soon as well. Their relationship had become so acrimonious that everyone expected them to break up. Instead, they got married. This kind of echoes the somewhat confusing story, with timelines changing between past and future, people walking through mirrors, lost empires, white voids, and intersecting universes. Romana was really quite impressive, and it was a pretty good send-off—rare for a companion.