The Doctor has a secret meeting with Vardan aliens, then takes Leela and K9 to Gallifrey, and claims the presidency, which is his right because of the events in the Deadly Assassin story. He orders the Presidential Suite lined with lead. During the induction ceremony, the Crown of Rassilon seems to reject him, and he is injured. Leela is accused of attacking him, though she was only trying to help him, and she is banished from the Citadel. Later, in front of the Time Lords, three Vardans materialize and he addresses them as the Time Lords’ masters. The slimy Castellan Kelner (Milton Johns) kowtows to them instantly, issues arrest orders to their benefit, and banishes some of the Time Lords. The Doctor tells his old friend Chancellor Borusa (John Arnatt) that the Vardans can read minds, but not through lead. The Doctor intends to put the dangerous Vardans in a time-lock on their home world but needs to do what he is doing to determine its location.
Leela is certain that the Doctor banished her for her own protection. With the banished Time-Lady Rodan (Hilary Ryan), she travels across the wastelands, where they meet a group of outsiders, led by Nesbin (Max Faulkner). Nesbin agrees to help take the Citadel back.
When the Vardans seem mistrustful, the Doctor offers to dismantle the Quantum Forcefield that surrounds and protects Gallifrey, which would allow the Vardan invasion force to enter. But he only creates a small hole in the forcefield, which allows K9 to trace the Vardans to their home world and engage the time-lock. Unfortunately, a squad of Sontaran warriors appear, led by Commander Stor (Derek Deadman), who reveals that the Sontarans have used the Vardans to seize control of the Citadel. Castellan Kelner switches sides without hesitation. The Doctor, Borusa, and their allies escape to join Leela and the Outsiders. Leela helps defend the Citadel with Andred (Chris Tranchell) while Rodan repairs the hole in the forcefield.
The Doctor needs the Key of Rassilon, and convinces Borusa to give it to him, so he can power the Demat Gun, which erases its target from all time. The Doctor uses the labyrinthine corridors of the inner TARDIS to elude the Sontarans while K9 and Rodan build the Demat Gun. With it, he goes after Stor, who is trying to destroy the entire galaxy. He fires the gun and the Sontarans disappear, as Stor and all his orders and plans vanish from time. The gun has kickback and the Doctor remembers nothing about them. As the Doctor prepares to leave, Leela announces that she wants to stay behind with Andred, and K9 wants to stay with her. After leaving in the TARDIS, the Doctor opens a box labelled K9 Mark II.
This story did not get much love at first--though it has gained respect since--mostly because of the Vardans and the Sontarans. The former were shimmering bits of tinfoil and I have seen worse; the Sontarans were a little too amusing to be frightening, but I have always found them highly entertaining. The fact that they kept tripping over things was explained as a result of their evolving on a high-gravity world, but mostly it was because they could barely see in their costumes. Most of Part Six was spent wandering through a labyrinth of corridors deep in the TARDIS—it was filmed in a disused mental institution—with the characters constantly finding themselves back where they had started, and I found it highly amusing. Gallifrey, with its millennia-old bureaucracy and over-the-top costumes is always a hoot. Louise Jameson had been offered the role of Portia in The Merchant of Venice at the Old Vic. She wanted Leela to die nobly and was disappointed that she stayed behind with a man though there was little indication of a budding romance. The two actors did what they could to give that impression. The producers thought her death would be traumatic for younger viewers. In my opinion, Leela was wasted. Her character, particularly with this Doctor, had untold promise for action and humor. But the companions generally—until recent years—seldom seemed to live up to their promise. Holmes without Watson, Frodo without Sam, Green Hornet without Kato. Too bad.