On the colony planet Frontios in the Veruna system, Captain Revere (John Beardmore) is knocked unconscious when some subsidence occurs in a mine. Security Chief Brazen (Peter Gilmore) fetches help, but Revere’s body has now vanished. Following this incident, Brazen orders the research room closed – despite the protestations of the scientist Range (William Lucas) and his daughter Norna (Lesley Dunlop), who feel that the investigations into the meteorite bombardment that the planet is suffering should continue. Revere’s son, Plantaganet (Jeff Rawle), takes over as leader. 

The TARDIS is hovering above Frontios, upon which the last surviving group of mankind is living following Earth’s collision with the Sun. The TARDIS is mysteriously affected by a meteorite storm and dragged down to the planet by gravity. The Doctor, Tegan and Turlough emerge to investigate. The colonists injured by the meteorite bombardment are being helped into the medical area, and the travellers – despite the Doctor’s earlier protestations that he cannot get involved – all help out. 

The Doctor needs better light in the medical facility. Tegan and Turlough are sent to fetch a portable mu-field activator and five argon discharge globes from the TARDIS but 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 (Jim Dowdall) 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. Plantaganet wants the Doctor killed, 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. 

Turlough discovers a passage under the research room and he and Norna explore, only for Norna to be captured by the creatures that live there. These are Tractators (George Campbell, Michael Malcolm, Stephen Speed, William Bowden, Hedi Khursandi). Turlough goes into shock at seeing them, as the experience dredges up a deep racial memory of the creatures invading his own planet. The Doctor and Range also find the tunnel, and then so does Tegan. 

After many narrow escapes from the Tractators, the Doctor and Tegan find their way to the creatures’ control centre. There, their leader the Gravis (John Gillett) explains that the Tractators have been on Frontios for 500 years. They brought the colony ship here 40 years ago, and have been causing the meteorite bombardments for the last 30 years. They need human minds and bodies to run their mining machines, and they plan to replace the current driver – Captain Revere, who is now dead – with Plantaganet. The Gravis knows of the Doctor and is very interested in his TARDIS. 

Meanwhile Brazen, along with his deputy (Alison Skillbeck), has been holding an enquiry with Range, Norna and Turlough. Brazen and Range head back into the tunnels, followed by Turlough. In their absence, the research room is attacked by an orderly named Cockerill (Maurice O’Connell) and a group of retrograde colonists who feel that the present order is breaking down and want their independence. 

When Turlough catches up with Brazen and Range, Range goes back to check on Norna’s safety. Turlough rescues Tegan, who has been held in a gravity field by a Tractator, and they then move on to rescue the Doctor. The Doctor tampers with the mining machine and causes the Gravis to suffer an electric shock. The creature falls unconscious and all the other Tractators then mill around in confusion. Plantaganet is rescued from the machine but, in a struggle, Brazen falls into the driver’s seat and becomes hooked up instead. The machine goes wild and crashes into a wall. 

The Doctor realises that the Tractators are building a gravity motor with which to pilot the planet out of orbit. Plantaganet reveals that there are in fact two mining machines, and Turlough remembers an important fact about the creatures: if the Gravis is removed from the others, they all cease to be intelligent and return to a docile state. 

Wandering in the tunnels, Tegan comes across bits of the TARDIS’s inner walls. She is chased by some Tractators led by the Gravis, who has now regained consciousness, and comes upon one of the TARDIS’s inner doors. Going through it, she finds herself in the TARDIS console room, which has bits of rock wall mixed in with its normal walls. The Doctor, Turlough and Plantaganet are already there. The Doctor ushers the Gravis in and then goads it into reassembling the TARDIS using its power over gravity. When the outer plasmic shell forms, the Gravis is effectively cut off from its 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. 

Once the TARDIS has left Frontios, its engines start making a worrying noise. The Doctor can do nothing. The ship is being pulled towards the centre of the universe.

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}