The Key to Time locator points to the cold and boring planet Calufrax, but when the Doctor and Romana (Mary Tamm) arrive in the TARDIS, they find a prosperous civilization. A band of people called the Mentiads are feared because of their mysterious mental powers. But the Doctor is more suspicious of the Pirate Captain (Bruce Purchase), whom he suspects is not the benefactor he claims to be, despite the precious jewels lying in the street like pebbles. It seems, in fact, that they are on a hollowed-out planet called Zanak, which materializes around other, smaller planets and plunders their resources.
Zanak’s engines were damaged as the planet and the TARDIS materialized in the same place. The Captain repairs the engines and gets ready for the next jump—to Earth. The Doctor discovers that the real menace controlling the blustering and bloodthirsty Captain is the Tyrant Queen Xanxia (Rosalind Lloyd), disguised as the Captain’s nurse, who is using the plundered resources to become immortal. Her ancient body, in fact, is stored in stasis within the ship, between two Time Dams that hold back the ravages of time. The younger version of herself is just a projection. The Captain is not as crazy as he seems and is calculating plans to destroy Xanxia. The Mentiads’ powers are enhanced by the deaths of the people on the destroyed planets.
The Key to Time locator has been giving off strange signals, as if the Key was everywhere. When the Doctor and Romana see the Captain’s trophy room, holding the tiny shrunken cores of the victim planets, they realize that the planet Calufax itself is the segment they are looking for. The Captain is planning to use the gravitational power of the crushed worlds to drill into the Time Dams, bypass the fail-safe, and let Time free to kill the Queen. The Doctor uses the TARDIS to disrupt Zanak’s materialization around Earth, while the Mentiads sabotage the engines with their telekinetic powers. Xanxia kills the Captain when his plan fails because Calufrax is not a real world—it’s a disguised Key to Time--and its mass is different. The Doctor, Romana, and the Mentiads destroy the planet’s bridge and Queen Xanxia. The Doctor uses the crushed worlds to fill up the hollow centre of Zanak and create a real world for the people to live on in peace.
An accident in the beginning, where the bad landing causes the Doctor to crash head-first into the TARDIS console was to explain his split lip, which was actually caused by Tom Baker playing with a friend’s dog in a pub. The rather impressive scenes in the ship-planet’s engine room were filmed at the Berkeley Nuclear Power Station in Gloucestershire. There is a recurring chase as K9 zooms up and down the corridors in pursuit of the Pirate Captain’s robot parrot, which he uses to assassinate people. They shoot lasers at each other until K9 finally brings the broken bird back to the Doctor like a retriever. Who’s a good robot? Much of the rest of the story is just as over-the-top because it was written by Douglas Adams just as his radio-play Hitchhiker’s Guide to the Galaxy was being picked up by BBC Radio. He worked on both projects simultaneously. The Great Parrot Chase idea was suggested by Adams’ ten-year-old brother, who was very proud until he had to sit for hours as the crew tried desperately to create the special effects necessary. Despite the misgivings of much of the Doctor Who writing staff, who thought Douglas Adams would take the series too deep into comedy and satire, he wrote a number of stories for Tom Baker, who relished the Python-like silliness. At one point, the Doctor says, “Don’t panic.” In turn, a line of the Doctor’s was used in the Hitchhiker’s Guide.