The TARDIS lands in the far future, on the planet Frontios, where some of the last vestiges of humanity are struggling for survival. The planet is being attacked by meteorite showers orchestrated by an unknown enemy responsible for the disappearance of several prominent colonists, including the colony’s leader, Captain Revere. After witnessing Revere being “eaten by the ground,” Security Chief Brazen engages in a cover up. To the public, Captain Revere died of natural causes. After a state funeral, Revere’s son, Plantaganet, assumes the leadership of the colony.
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 render 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, seemingly destroyed; all that is left is the Doctor’s hat stand.
Plantaganet orders the execution of the Doctor, but Turlough intercedes, using the TARDIS hat stand 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 Plantaganet 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. Turlough briefly undergoes a sort of nervous breakdown due to the fact that the Tractators once attempted to invade his home world long ago; his mind contains a deep, horrific "race memory" of the event. The disappearing colonists were being used by the Tractators to run their mining machines. Plantaganet was kidnapped to replace Captain Revere, the current driver who 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.