Captain Revere, the leader of the human colony on Frontios, has led an expedition of Orderlies into the tunnels beneath the planet's surface to study the minerals there. As he does so, the ground itself begins to move beneath him, and the tunnels cave in when a support beam shifts in place. Revere is buried beneath the rubble, and Chief Orderly Brazen tries to rescue him -- only to find, when the dust clears, that Revere's body has vanished. Unnerved, Brazen decides to cover up the true facts of Revere's death and claims that he was killed by an explosion in the science room. The science room is thus sealed off, much to the disgust of the colony's chief scientist and medical officer, Mr Range.

While the Doctor tries to work out where to position the TARDIS hatstand, his ship drifts far into the future and comes to rest around Frontios -- one of the last outposts of human civilisation following the collision of the Earth with its sun. The Doctor refuses to put down, as these times mark the far limit of Gallifreyan knowledge; as the colony is still in its infancy any interference could prove disastrous. But before he can depart the TARDIS is swept up in a meteor shower -- and, much to his surprise, his ship is dragged down to Frontios by the planet's gravitational pull.

The meteorites bombard the colony, and the people scatter in panic as klaxons sound. The TARDIS materializes as the bombardment begins to die down, and the Doctor instantly rushes to the aid of an injured colonist. Turlough and Tegan help Norna, Range's daughter, to carry the injured to the medical shelter, where Range is surprised by the presence of the three strangers. The Doctor is taken aback by the primitive state of the medical facilities, lack of basic medical technology and the almost non-existent lighting. Despite the rules against interference the Doctor tells Tegan and Turlough to fetch the mu-field generator from the TARDIS so he can at least provide Range with decent lighting. But Tegan and Turlough find the inner door of the console room jammed in place, as if twisted out of shape by some powerful force. Meanwhile, Brazen reports the strangers' arrival to Revere's son, Plantagenet, and both fear that this could herald the beginning of the invasion they've expected for so long.

The Doctor doesn't pay attention to Tegan and Turlough's story when they return; he's fully occupied by Range's explanation of how the colony ship crashed on Frontios. All of their technology failed and was destroyed in the crash, and after a decade spent tilling the fields the bombardments began. Although there is little doubt that the colonists of Frontios are being deliberately attacked, nobody yet knows why, or by whom. Tegan and Turlough, meanwhile, discuss the lighting situation with Norna, who offers to help them break onto the colony ship and steal an acid battery from the science room, in order to run more power through the weak phosphor lamps.

As Brazen and Plantagenet emerge from the colony ship, Norna helps Tegan and Turlough to sneak on board -- but they are spotted, and Brazen follows them with a squad of Orderlies. They manage to evade the Orderlies and get into the science room, where they use a block and tackle to lift the heavy battery out onto the hull of the ship. Unfortunately, their escape route is blocked; an Orderly named Cockerill has stolen extra food from the ration stores, and he and his fellows have settled in for an illicit meal. Norna decides to lower the acid battery over the side of the ship, but as they near the ground they are spotted by the Warnsman, who normally keeps an eye out for signs of the bombardment. He rushes over to confront them and is accidentally knocked out by the swinging battery -- as the skies begin to darken...

The Doctor, still trying to come to terms with the loss of the TARDIS, is surrounded by an impromptu firing squad of Orderlies. Norna stands in the line of fire, pointing out that even if the Doctor is an invader he will at least know why the colony is under attack. When the Doctor is unable to answer the question immediately, Plantagenet orders his men to open fire. Turlough grabs the nearest weapon, which happens to be the TARDIS hatstand -- and a small explosion scatters rubble about the area. The Doctor identifies the effect as the result of residual energy from the TARDIS, but Plantagenet and Brazen are convinced that the hatstand is in fact a weapon -- perhaps the very one which brings down the bombardment.

Turlough uses the hatstand to hold the Orderlies at bay, while the Doctor offers his help to find answers. Revere spent his life trying to find precious minerals beneath Frontios, but the Doctor intends to analyse the meteorites instead and find out where they're from. Plantagenet and Brazen reluctantly accompany the Doctor back to the science room, held at bay by the hatstand -- but once in the science room Plantagenet tries to strike Turlough down with a crowbar. Turlough swings around with the hatstand, and Plantagenet, although untouched, collapses, clutching his heart. The Doctor puts an end to the farce and checks out Plantagenet -- who was struck a glancing blow during the bombardment and has been trying to carry on regardless. The Doctor accompanies Brazen and Plantagenet to the medical shelter while Turlough and Norna begin analysing the meteorite fragments.

Tegan, meanwhile, has been helping Range to set up the new lighting system with the acid battery, and in the process learns more about the disaster that is Frontios. Colonists regularly desert the failing colony to live retrograde lives out in the wilderness, and the Orderlies have begun to shoot deserters -- although each death increases the possibility of the colony's extinction. Tegan notices that Range is keeping a set of files about deaths, and while Range is distracted by an emergency Tegan studies the files and finds one marked "Deaths Unaccountable". Range catches her and orders her not to mention the file to anyone. At that moment, the Doctor arrives with Brazen and Plantagenet, and uses the acid battery jar as a defibrillator to save Plantagenet's life. Brazen still isn't sure of the Doctor's intentions but is more willing to trust him. Plantagenet is still weak, but refuses to leave the medical shelter for his quarters on the colony ship; he must stay in full view of the colonists, for if they believe he is dead anarchy will result.

Turlough wonders why the colonists don't dig underground shelters, but Norna claims that Revere outlawed digging beneath the earth. Turlough realizes that some of Revere's rock samples are labelled with dates from after the quarry was supposedly closed, and eventually deduces that the block and tackle which they used earlier with the acid battery is in fact there to lift up the floor plates. Beneath the science room he and Norna find secret tunnels in which Revere continued his subterranean exploration and analysis. The rock in the tunnels is strangely moth-eaten, and as they continue on they find an area in which the walls are polished as smooth as glass. Turlough finds it all unnervingly familiar, and realizes that a word, or name, is appearing in his memory... but where has he heard the word "Tractators" before? He wants to go back, but Norna insists that they continue on -- unaware that they are being followed by two creatures like giant woodlice...

The Doctor and Range return to the science room, leaving Tegan and Brazen in the medical shelter. Tegan is coming to understand Brazen's difficult position, but while talking with him she accidentally lets slip about the existence of Range's secret files. Brazen concludes that Range is the one spreading rumours and sedition, but as he calls the Orderlies together to take action, Tegan hears Plantagenet crying out for help -- and arrives just in time to see him sink down through the ground and disappear. At that moment Cockerill arrives with a group of colonists, and Brazen bluffs them away, claiming that Plantagenet has better things to do than appear for their benefit. Tegan, realizing things are getting out of control, slips away back to the colony ship -- only to find that the colonists are already looting it. The anarchy which Plantagenet had feared has begun.

Tegan throws her phosphor lamp to the ground, and the burst of light and fire distracts the Tractators and breaks their grip on the Doctor and Norna. Tegan gets Norna to safety while the Doctor keeps the Tractators occupied, but she then returns for the Doctor and is captured by the same gravity beam which is holding him. Range arrives to see the Doctor and Tegan being dragged away by controlled gravitational forces, and the Doctor warns him to get Norna and Turlough to safety. As the Tractators approach, the Doctor throws his own phosphor lamp at them, breaking their hold on him; he and Tegan then flee into the tunnels and manage to shake off the pursuing Tractators.

Brazen attempts to restore order in the colony, but even Cockerill has gone Retrograde, claiming that Frontios is finished. Brazen turfs Cockerill out of the colony ship with his stolen food, and doesn't lift a finger to help him when he is set upon by other Retrogrades and beaten nearly to death for his rations. Lying helpless on the ground, Cockerill is drawn down through the earth as the Retrogrades watch -- but in the tunnels below ground, the Doctor and Tegan stumble across the Tractator responsible, and are forced to flee when it abandons its former quarry and turns its attention to them. When Cockerill rises from the earth, the Retrogrades are amazed -- surely a man who can free himself of the hungry earth can do anything.

As Norna and Range carry Turlough back to the surface, he continues to babble about the Tractators and their invasion of his homeworld, as deep ancestral memories surface from his unconscious. He refers to them as an infection of the planet -- and their intentions are evil. As they near the entrance to the colony ship, Norna finds a shred of map with Revere's handwriting on it -- proof that he knew or suspected the presence of the Tractators. Upon arriving at the science room, however, Range and Norna are arrested by Brazen, who accuses Range of spreading disinformation to subvert law and order on Frontios. Range has been collecting facts and statistics to prove that the legends of "the hungry earth" are true -- and it now becomes clear that, while Revere kept his suspicions secret to avoid panic until he knew the true nature of the threat, Range was keeping the vital information secret because of his leaders' conspiracy of silence.

Turlough slowly begins to recover from the shock of his ancestral memories surfacing, and when Brazen admits the true facts about Revere's death, Turlough informs him that Revere and Plantagenet may still be alive. The Tractators need living minds as well as bodies to carry out their plans. Brazen decides to lead the Orderlies into the tunnel system to rescue his colony's leaders, and Range offers to guide them while Norna remains with the still-recovering Turlough. Turlough, ashamed of what he now sees as his cowardice, eventually sets off after the expedition to help them -- unaware that Retrogrades are on the loose in the colony ship, and that he's left Norna alone in their midst...

The Gravis seems to know the Doctor by reputation, and is intrigued when Tegan inadvertently reveals that they travelled to Frontios by TARDIS. As the Tractators have been marooned on Frontios for centuries, the Gravis is obsessed with means of travel. The Doctor seems to side with the Gravis, pointing out that the Tractators were here long before the humans arrived; Tegan is furious with him, but the Doctor informs the Gravis that she's a malfunctioning android and the Gravis has a Tractator pinion her in a side cave out of the way. Captain Revere has finally died, and the Gravis is now preparing to install Plantagenet in his place. The Doctor has by now worked out that the colony ship's crash was caused by the Tractators, who allowed the colonists a decade to establish themselves before beginning the bombardments, using their control over gravity to draw the system's asteroids down upon the planet. He has also concluded that the polished tunnel system is intended as a wave guide to amplify the power of the Tractators' gravity beams. What he doesn't yet know is why...

Turlough catches up with Brazen and the others, and Range returns to the colony ship to ensure that Norna is all right. The Orderlies reach the Tractators' caverns, where they rescue Tegan and rush into the main cave. The Doctor takes advantage of the distraction to release Plantagenet from the excavating machine, and when he breaks the link an electrical charge earths itself through the nearby Gravis, stunning it. The Tractators are unable to function independently without the Gravis, and in the confusion which follows the Orderlies attempt to retreat. Turlough, however, is mesmerised by the excavation machine, which symbolises all that is evil about the Tractators in his unconscious memory. Brazen manages to pull Turlough away from the machine but is himself caught by its thrashing linkages. He orders the Doctor to get the others to safety as the machine runs out of control and begins to tear him apart.

Range is captured by a Tractator on his way back to the colony, but escapes when it is thrown into confusion by the loss of the Gravis. Meanwhile, Norna is attacked by a Retrograde, but is rescued by Cockerill -- who is now the leader of an army of Retrogrades trying to take command of Frontios and restore order. They don't believe Norna's wild story about Tractators until Range arrives, terrified and shaken by his experiences, and convinced that Frontios is doomed.

The Doctor, Tegan, Turlough and Plantagenet shelter in the nearby tunnels, and Plantagenet reveals that he knows the Gravis' plan; the extended tunnel system is to act as a gravity motor, enabling the Tractators to pilot the entire planet throughout the galaxy and conquer other worlds. And Turlough has remembered the Tractators' greatest weakness; the Gravis is the source of all their power, and once separated from it they will become harmless burrowing insects. The Gravis recovers, and they are forced to flee into the tunnels -- and the Gravis, realizing that Tegan is not an android after all, decides to use her as the new driver for the excavating machine.

Tegan finds parts of the TARDIS interior scattered through a nearby tunnel, and when she ducks through a door to avoid the Gravis she finds herself in the remains of the TARDIS console room. The Doctor has also located it and is using the last vestiges of power in the console to set a trap for the Gravis. He emerges into the tunnel and offers to surrender to the Gravis, but "accidentally" lets the Gravis catch a glimpse of the console room behind him. Obsessed with the power of travel, the Gravis uses all of its strength to pull the scattered remains of the TARDIS back together -- but once the TARDIS has been reassembled and the outer plasmic shell locks into place, the interior dimensions once again become separate from the exterior, and the Gravis is cut off from the other Tractators. Already exhausted by the effort of reassembling the TARDIS, the Gravis sinks into a coma.