The TARDIS materializes on an ancient space station. Sarah (Elizabeth Sladen) is overcome by the low oxygen levels at first, while Harry (Ian Marter) and the Doctor (Tom Baker) explore. Sarah is transported into cryonic suspension by the computer. The others concludeme to the conclusion that the station is a kind of Ark, filled with humans who have been frozen for millennia. Harry finds Sarah and searches for some way to resuscitate her but discovers a giant mummified insect.
A woman called Vira (Wendy Williams) comes out of suspended animation. She revives Sarah and Noah (Kenton Moore), the leader of what is called Space Station Nerva. The Doctor tells Vira that Nerva’s crew has overslept a bit—several thousand years—thanks to the insect visitor who sabotaged the station’s systems. Noah accuses the visitors of murdering a missing crewmate. Most Doctors have been accused of crimes when they land somewhere, but the Fourth Doctor made a habit of it.
Investigating the power room, Noah is infected by an alien creature called a Wirrn. The Doctor realizes the alien insect they found laid eggs inside the missing crewman. Vira is ordered to revive the remaining crew and evacuate the station, but the Doctor realizes the alien pupae will mature too quickly and they must destroy the Wirrn in their dormant pupal stage. Dissection of the Wirrn corpse reveals they are vulnerable to electricity. As he tries to power up the station, Noah, as a more fully developed Wirrn, attacks him.
The Doctor plans to electrify the cryogenic chamber to keep the creatures out. Because the Wirrn have disabled the station, the crew decides to use the generators on a transport ship docked at the station—Sarah’s idea, actually. She has to crawl through a narrow conduit to carry the power cable from the ship, and the Doctor manages to electrify the cryogenic chamber. Noah, now the Wirrn swarm-leader, offers safe passage to the revived if they leave the sleeping crew for them, but they crew declines.
Noah leads the swarm in an assault on the transport ship, Vira and the crew set the autopilot and escape the ship. The transport carries off the entire swarm. The Doctor wonders if this was not Noah’s plan all along, motivated by some shred of humanity inside. Noah sends one final good-bye to Vira before the transport explodes. The TARDIS crew teleport down to Earth to repair the receiver terminal so the colonists from the ark can repopulate the Earth.
The story, produced by Philip Hinchcliff, was a respectable science-fiction tale and is thought in some quarters to have influenced Alien. Parts of the station were brightly lit, and parts were dark and moody, like a meeting of 2001 and Alien. Both seemed realistic. The adult Wirrn were not that great, as Doctor Who Inc. never really solved the problem of putting a giant insect body on a two-legged actor. But the larval stage made use of a brand-new product called bubble wrap, which they painted green and wrapped around poor Noah as he was consumed by the creature. In fact, his terror and despair as he begs Vira to kill him—probably the best acting in the piece—was left on the cutting room floor because the studio thought it would be too upsetting to children. The biggest problem with the bubble-wrap was that it was constantly popping and had often to be filmed without sound. The Doctor’s scarf was damaged by a laser and he was saddened, as it had been knitted for him by Madame Nostradamas. The real scarf had been knitted by a little old British lady named Begonia Pope who was given an entire trunk of yarn and used it all, producing an 11-foot scarf. Tom Baker loved it and used it and its successors for seven years of stage business.