The TARDIS lands on the planet Frontios in the far future, where the last human survivors are hanging on by a thread. An unknown enemy is directing meteorite showers at them, and Captain Revere, the colony’s leader, is seen being eaten up by the ground. Security Chief Brazen (Peter Gilmore) keeps the fact secret, saying he died of natural causes, and his son Plantagenet (Jeff Rawle) becomes the leader. The TARDIS is pulled down by gravity and the Doctor and his companions emerge, in the middle of the bombardment, to investigate. The Doctor violates Time-Lord rules (No, really?) to help the injured colonists.
Needing better light, he sends Tegan and Turlough to fetch it from the TARDIS, but they find the inner door will not open and they only have access to the Console Room. But they obtain lights in the colony’s Research Room. As they return, they are forced to knock out Norna the Warnsman (Lesley Dunlop) to avoid capture. Another bombardment happens, without warning for lack of the Warnsman. The colony is damaged, and the TARDIS is destroyed entirely, except for the Doctor’s hat-stand sticking out of the rubble. Plantagenet orders the Doctor’s execution, but Turlough defends him by brandishing the hat-stand. Plantagenet has a heart-attack and the Doctor treats him, but Plantagenet is dragged into the ground.
The Doctor, Turlough, and Tegan discover the culprits are giant insects with power over gravity, led by the Gravis (John Gillett). Turlough is disturbed because his home planet was infested by the creatures long ago and he has a deep race memory of the fact. The disappearing colonists are running the Tractators’ mining machines, their bodies encased in the machines and their brains wired to the controls. Captain Revere is brain dead by now, and Plantagenet is next to be connected. Gravis and the Tractators (not the name of a rock-band) want to turn Frontios into a giant spaceship to spread terror throughout the Galaxy (what else?). The Doctor and his little army knock out the Gravis and rescue Plantagenet, but Brazen is caught by a mining machine and killed.
Tegan wanders in the tunnels and finds bits of the TARDIS. She is chased by the Tractators, finds one of the inner doors and opens it, finds herself in the Console Room, and the Doctor is inside. He tricks the Gravis into re-assembling the TARDIS and it restores itself. The Gravis is trapped and the Tractators rendered harmless. The now-dormant Gravis is to be dumped on the uninhabited planet Kolkokron. But as the TARDIS leaves, it is pulled toward the centre of the universe.
The designer Barrie Dobbins committed suicide, though probably not because the creatures were so badly made. One of the actors, Peter Arne, known from Victor/Victoria and Return of the Pink Panther, was bludgeoned to death in his flat. His supposed killer, his flat-mate, was found dead in the Thames. This kind of cast a pall over the whole project. The Tractators were inspired by woodlice, which had infested the scriptwriter’s apartment. Dancers were hired to play the monsters, as they could twist and writhe like woodlice, but the costumes made this impossible. Turlough was sent to get a portable mu-field generator and some argon discharge globes.
The creatures, which could have been terrifying, were wooden and unreliable. The human-powered mining machines, which could have been horrible, looked like a Rube Goldberg device. Being sucked down into the earth below your feet as you are bombarded by meteors from above sounds like something from one of the Circles of Hell. If the human-powered mining machines had been built as they were designed, they probably would have scared the children. The struggle of the Last Survivors of the Human Race could have been filled with pathos. Instead, it all seemed kind of pedestrian. Peter Davison, however, is quite good. He seems to be relating strongly to his companions just as they are about to leave the TARDIS.