The Doctor follows a telepathic cry of pain to the planet Kastopheria, otherwise known as Catastrophea. In a crowded human colony city, he and Jo see a man beating his native bearer, and when they intervene, the ensuing fight leads to their arrest. The abusive human, Councillor Rekar, intends to use his influence to have the Doctor and Jo sentenced to life on a labour farm, but their case is heard by the sympathetic Administrator Charteris, who fines Rekar for his assault on the native and gives the Doctor a suspended sentence. By the time the Doctor and Jo emerge from the courtroom, they've already been made symbols of the revolution by the young idealist Makos and his friends, who believe the Doctor to be the revolutionary El Llama.

The Doctor inquires about the political situation, and soon learns where the planet acquired its nickname; it's a catastrophe waiting to happen. The planet was originally colonised by a pharmaceuticals company which oppressed the pacifistic natives, but when corruption and drug smuggling became rampant the United Planetary Assocation took over. The natives use the drug skar in their religious ceremonies, but their temples are being raided by smugglers, as skar creates a temporary high in humans, followed by a psychotic crash. The UPA has recently become distracted by the threat of war from the Draconians, and the company is attempting to make a comeback; Rekar is already assembling a private army on his plantation. Catastrophea has been blockaded by the Draconians, who seek to annex the planet for their empire. The city police, company guards, and UAP troops don't trust each other, the young revolutionaries want freedom for the People, and the People themselves have recently begun going berzerk and killing at random for no reason...

The Doctor decides to leave, as the planet's problems are too complex for him to solve. But on their way back to the TARDIS he and Jo are attacked by Rekar's hired goons, as Rekar seeks revenge for his earlier humiliation. The Doctor and Jo overpower him but are then arrested by UAP peacekeepers, on the orders of General Walton. Walton has also concluded that the Doctor is El Llama and decides to execute him before he can stir up further trouble. The Doctor nearly manages to convince Walton that he's made a mistake, but the young revolutionaries hear of his capture and attempt to rescue him. The Doctor is forced to flee with them when the soldiers open fire, and Walton sends out more patrols with orders to shoot him on sight. One such patrol locates the TARDIS, which is taken back to the stockade, and the Doctor realizes that he now has no choice but to solve Catastrophea's problems in order to recover his ship.

The Doctor, certain that the People's civilisation can't be as idyllic as it appears, decides to begin his investigation with their temples. He soon meets a telepathic priest named Zylor, who greets him as the hero predicted in their prophecies and explains the People's real dilemma to him. But the Doctor isn't sure how -- or even whether -- he should help them. Meanwhile, the revolutionary Garon, jealous of the Doctor's growing influence over his former followers, decides to steal back the TARDIS in order to force the Doctor to do things his way. But the UAP troops have bought out the revolutionaries' informant, and he leads them into an ambush which results in Garon and Jo being captured. Walton posts messages about town threatening to execute Jo unless the Doctor surrenders. Meanwhile, a Draconian scoutship lands in the jungle, to provoke the Earthmen into attacking and thus justifying an invasion.

The Doctor turns himself in, and tries to warn Walton and Charteris that the real crisis involves the People and places the entire planet at risk. When Walton refuses to listen to him, the Doctor sends a telepathic message to the People, who gather at the gates of the army base and push their way in. While the guards are distracted, the Doctor and Jo slip out of the base through the back. Walton and Charteris discuss the Doctor's enigmatic comments; are the berzerkers the real crisis? The incidents do seem to be increasing in frequency... and what would happen if all the People on the planet went berzerk at once?

The criminal kingpin Dove has noticed the Doctor associating with the People and exploring the jungle. The drug smugglers have nearly tapped out the People's supplies of skar, but it's rumoured that there's a giant stash hidden somewhere in the jungle -- and Dove, believing that the Doctor has learned where it is, follows him and Jo on their expedition to the People's secret temple. On their journey, the Doctor explains the situation to Jo. The People used to be a barbaric, violent species until the priests used skar and telepathy to control their savage impulses. Unfortunately, they went too far, and the People have been trapped in a pacifistic mindlock for centuries, unable to defend themselves against outside threats such as the human invasion. Worse, thanks to the drug smugglers' theft of skar from the temples, the mindlock is deteriorating, resulting in random berzerker incidents. Unless the mindlock can be broken by an outsider, such as the Doctor, the People will gradually decline back into savagery.

The Draconians grow tired of waiting for the humans to notice them, and send out a patrol to provoke an incident. The patrol encounters Dove, who kills them, and Captain-Lord Samzyre takes personal command of the follow-up expedition. The Doctor and Jo, meanwhile, arrive at a secret temple built over the crater of a dormant volcano, where the People's priests focus their mental energies on a giant crystal of skar -- the Anima, the soul of the People. The Doctor must destroy this to save the People, but if he does, what will stop them from reverting to barbarism forever? Before he can decide, Dove arrives and tries to steal the skar for himself, but the Draconians arrive and open fire, killing Dove -- and destroying the Anima.

Samzyre assaults one of the priests, believing that he's found a secret human military base, but the priests turn on him and his followers and nearly kill them all before Zylor brings them back under control. The Doctor advises the surviving Sergeant Korr to evacuate and let the crisis play itself out. Zylor and the priests try to moderate the damage while the Doctor and Jo return to Cat City. Once there, they learn that panic is spreading as the humans slowly learn that the People are no longer taking abuse. Councillor Rekar's plantation is burned to the ground by rebelling servants during his daily flogging ritual, and, furious, he returns to Cat City to assemble a squad of Company guards.

Sergeant Korr returns to the mothership and reports to General-Baron Kryll, but Kryll doesn't accept his interpretation of events. Since everyone knows the People are pacifistic, Samzyre obviously just stumbled across a minor cult uprising -- proof that the Earthmen can't control the native populace. Kryll's influence at court is growing, and if he successfully adds Catastrophea to the Draconian Empire he may be able to topple the Emperor and begin a new dynasty. He thus uses the incident at the temple as an excuse to invade.

The Doctor explains the situation to Charteris and Walton and advises them to evacuate, but they refuse; they are unwilling to walk away from their duty to protect the planet for Earth, and besides, there aren't enough ships to evacuate every colonist. The People arm themselves with the weapons they abandoned centuries ago and march on Cat City, and Charteris is forced to free Garon and ask his revolutionaries to help protect the perimeter without provoking the People unnecessarily. The Doctor hopes for enough time to convince Charteris to evacuate and to convince the People to give them the time they need, but the fragile stalemate is broken by Rekar, who leads his guards out of the city to attack the People. He realize too late how far things have gone; the People slaughter his guards, kill him, and advance on Cat City...

Just as Walton is about to give the order to attack, Kryll's ship lands, distracting the People. Kryll and his soldiers march out of their ship, ignoring the natives -- until they attack. On the Doctor's advice, Charteris and Walton rescue Kryll, and once safely inside the city walls, Kryll -- shaken badly by the slaughter of his troops -- accepts the Doctor's explanation of events. The Doctor presses home his point; freed of their mindlock, the People will die rather than surrender. The only way the humans or Draconians can control Catastrophea now is by committing genocide. Kryll and Charteris have no choice but to draft a treaty, pledging non-interference in the affairs of the planet. Walton offers to have his troops escort Kryll back to his ship, and Kryll offers the services of his fleet to evacuate the human population.

The Doctor telepathically contacts Zylor, who agrees to the terms; the colonists will be allowed to evacuate, and the People will be left alone to find their own destiny. The Doctor and Jo return to the TARDIS and depart, hoping that the People will find peace some day and that this event will signify a new era of co-operation between Earth and Draconia. As the evacuation begins, Garon and Makos are surprised when a stranger arrives, identifies himself as El Llama and asks how he can help...