The TARDIS is mysteriously affected by a meteorite storm and dragged down to the planet by gravity. The Doctor, Tegan and Turlough emerge, in the middle of the bombardment, to investigate. Despite his earlier reservations about getting involved, the Doctor violates the cardinal rule of the Time Lords by helping the colonists who were injured by the meteorite bombardment and by providing medical assistance.
Needing better light in the medical facility, the Doctor sends Tegan and Turlough to fetch a portable mu-field activator and five argon discharge globes from the TARDIS. However, once they arrive, they find that the ship’s inner door is stuck, preventing them from getting beyond the console room. Norna, Tegan and Turlough obtain an acid-battery from the research room to power the lights. On their way back, however, they are forced to knock the Warnsman unconscious to avoid capture. Another bombardment occurs and, in the Warnsman’s absence, catches the colony unawares. When the skies clear, the TARDIS has gone and all that is left is the Doctor’s hat stand.
Plantaganet orders the execution of the Doctor, but Turlough intercedes, using a surviving hat stand from the TARDIS as a weapon. Plantaganet tries to attack the Doctor with a crowbar but suffers a heart attack. The Time Lord manages to save his life using the battery, but he is later dragged into the ground by some mysterious force.
The Doctor, Tegan and Turlough discover that the culprits are the Gravis and his Tractators, giant insects with incredible powers over gravity. The disappearing colonists where being used by the Tractators because they need human minds and bodies to run their mining machines. Plantaganet was kidnapped to replace Captain Revere, the current driver whose is now brain dead. The Gravis intends to transform Frontios into an enormous spaceship; once successful, he would be able to spread the terror of the Tractators across the galaxy. The Doctor, Turlough, Brazen and his guards rescue Plantaganet by knocking out the Gravis. However, Brazen gets caught by one of the mining machines and is killed while the others escape.
Tegan wanders around in the tunnels and comes across bits of the TARDIS’s inner walls. She is chased by the Gravis, who has now regained consciousness, and two of his Tractators. She inadvertently comes upon one of the TARDIS’s inner doors and she opens it to find herself in the TARDIS console room, which has bits of rock wall mixed in with its normal walls. She also finds the Doctor, Turlough and Plantaganet hovering around the console. The Doctor ushers the Gravis in and then tricks him into reassembling the TARDIS by using his power over gravity. The Gravis pulls the TARDIS back into its normal dimension. Once fully assembled, the Gravis is effectively cut off from his fellow Tractators, which revert to a harmless state.
The Doctor and Tegan drop the Gravis off on the uninhabited planet of Kolkokron. Returning to Frontios, the Doctor gives Plantaganet the hat stand as a farewell token and asks that his own involvement in the affair not be mentioned to anyone especially the Time Lords. Once the TARDIS has left Frontios, its engines start making a worrying noise. The Doctor appears to be helpless as the ship is being pulled towards the centre of the universe.