A group of musicians are praised by the Pilot (Peter Jeffrey), one of the leaders of their planet's seemingly idyllic colony, who tells them to keep practising for the forthcoming competition. Suddenly a man rushes past. Chief of Police Ola (Gertan Klauber) reports that Medok (Terence Lodge) has escaped from treatment. The Pilot orders guards to recapture him. 

The TARDIS materialises and the travellers disembark cautiously, knowing they will meet up with the clawed creature they saw on the time scanner. Medok crashes into them and Ben holds him until Ola and the guards arrive. Ola thanks the Doctor and his friends for helping to catch such a dangerous criminal while Medok is dragged off to the rehabilitation centre, screaming that he knows the secret behind the colony. At Ola's insistence the Doctor and his friends are taken to the refreshment room to receive the personal thanks of the Pilot. A still picture of the colony's Controller (Graham Leaman) appears on a screen. The voice of the Controller thanks them again and orders everyone in the colony to enjoy themselves. The travellers are invited to sample the many healthy therapies available, including massage and steam baths. 

Later the Doctor sneaks away from a reception being held in the travellers' honour and makes his way to the rehabilitation centre, where he sets Medok free. He wants to know more about the creatures Medok mentioned, but the terrified man runs off. The Doctor mentions the creatures to Ola and is warned that such talk will land him in the hospital for treatment. 

The Controller announces that Medok has escaped but assures the colonists that he will be recaptured. He entreats them to work harder, telling them that it is a privilege to refine gas for the colony. 

The travellers are told that a curfew comes into effect during the hours of darkness. They are ordered to go to rooms prepared for them in the refreshment centre. The Doctor however sneaks out and manages to locate Medok, who tells him that the crab-like creatures move up to the surface at night and any colonist who sees them is taken for treatment at the hospital. The guards close in and as the Doctor and Medok flee one of the creatures appears in their path. Medok tries to show the creature to a guard, but he refuses to acknowledge its existence. The Doctor and Medok are arrested and dragged off to face the Pilot. 

The Pilot is about to pass sentence on the Doctor for breaking the colony's laws when Medok is brought in and swears that the stranger was trying to persuade him to give himself up. The Doctor is thus absolved of any crime. Medok is taken back to the hospital and the Pilot tells the Doctor that the man will undergo treatment so that he can be happy like the other colonists. The Doctor asks why he wants everyone to be the same. He replies that the colonists' ancestors from Earth believed in the ideals of healthy happiness. After the Doctor is led off back to his quarters the Controller appears on the screen and orders the Pilot to have the travellers conditioned to think like the rest of the colony using deep sleep thought alteration. 

The sleeping Ben, Jamie and Polly are put into a hypnotic state by machines hidden in their quarters. A voice tells them to follow the rules of the colony, not to question anything and to work hard. It also tells them that they do not to believe in the Macra. Ben is fast asleep and heavily affected. Jamie however is restless and wakes. He rouses Ben and asks him if he heard a voice. Ben tells him to go back to sleep as they have a lot of work to do in the morning. Jamie is confused by this but is sent back to sleep when the hypnosis machine releases gas into the room. Early the next morning the Doctor wakes Polly. He warns her that someone has been trying to hypnotise her in her sleep. He tells her not to follow the orders she was given. Luckily Polly is unaffected. The Doctor and Polly rush to their friends' room. Jamie is also unaffected but Ben has succumbed. The Doctor destroys the hypnosis machine and a disgusted Ben rushes off to report his friends to the guards. The guards arrive and arrest the Doctor and Jamie. 

In the hospital Medok is not responding to the hypnosis treatment and is sent to dig for gas in the pits for the rest of his life. 

Polly goes in search of the Doctor and Jamie. Ben follows, telling her to obey the orders of Control. She tries to lose him but he catches up with her just as she comes across a Macra creature. Polly screams for help but Ben refuses to acknowledge the creature's existence, stating that there is nothing bad in the colony. More creatures appear, surrounding them. The sight of Polly in danger temporarily breaks Ben's conditioning and he rescues her. 

The Doctor and Jamie are brought to the office of the Pilot, who charges the Doctor with sabotaging equipment. The Doctor notices that even here there is a hypnosis machine. Ben and Polly are also ushered in. Polly is confused and upset when Ben again denies the existence of the Macra, but the Doctor tells her that the young man is still brainwashed by the evil influence that controls the colony. The Pilot assures him that the Controller is a good man. At that moment the usual still picture of the Controller appears on a screen, while a voice tells the Pilot that Control knows what has happened. The Doctor insists on seeing the Controller in person and the still picture is replaced by an image of a haggard middle-aged man, who feebly repeats the previous message. Suddenly a claw appears at the edge of the screen, terrorising the man. The screen quickly goes blank and the voice orders that the Doctor, Jamie and Polly be taken to the gas fields as slave labour. Once they have gone the voice orders the Pilot to forget what he has seen. The Pilot then orders Ben to follow his friends and spy on them for the colony. 

In the gas pits Jamie meets up with Medok, who warns him that life here will be short as the gas is poisonous to humans even though they have been given masks. Ben arrives to supervise the diggers and spy on his friends. The Doctor questions him about the voices he hears, trying to break down his conditioning. Suddenly a siren is sounded as a pocket of gas is uncovered. Everyone dons their masks. In the pumping room the machinery draws in the gas. Medok tells his new friends that although the whole colony is devoted to digging up and pumping the gas to the surface, no-one knows what it is for. The Doctor works out the formula for the gas on a blackboard in the pump room but a horrified Ola orders him to rub it off. 

Jamie manages to pilfer Ola's keys and uses them to unlock a door that he has discovered. Ben sees this but is beginning to regain his self-control and does not give Jamie away. Ola notices that his keys are missing only when an alarm sounds and the voice of Control announces that Jamie has entered a forbidden area. Ola prepares to send his guards after the young Scot but the voice refuses permission for anyone else to enter. Control orders that gas be pumped into the tunnels and the Doctor realises that this is being done to allow the Macra - who need the gas to breathe - to search for Jamie. He tells Polly that the Macra normally live deep underground but when the humans arrived they ventured to the surface, taking control of the colony and hypnotising the colonists to dig out the gas that they needed in order to survive there. 

Gas floods into the tunnels and Jamie begins to cough. In the pumping room the Doctor hastily traces the pipes until he finds the one pumping the gas and shuts it down. He then manages to reverse the flow, pumping fresh air down into the tunnels. He and Polly lock the door of the pump room and disappear into the forbidden area themselves. The guards break down the door and the voice of Control orders them to restore gas to the tunnels immediately. 

Jamie emerges from the tunnels into the middle of a group of dancers practising for the 'Happy Colony Finals'. To avoid suspicion he is forced to dance a Highland fling, but Ben gives him away to the guards. 

The Doctor and Polly have found their way to the Macra-infested Control centre deep in the tunnels. They decide that they must bring the Pilot to see it. 

The voice of Control orders all the colonists, including the Pilot, to dig for gas. The Doctor, Jamie and Polly are to be arrested on sight. The Pilot prevents the guards from taking the Doctor and agrees to go to the Control centre with him. There he sees the Macra and agrees that the colonists must overthrow them. The Macra then order the arrest of the Pilot and place Ola in charge of the colony. 

When the travellers and the Pilot return to the surface they are arrested by Ola and his guards. Control tells Ola to lock the prisoners in the pump room, which they will then fill with gas. Ben, now fully recovered, comes to his friends' rescue. The Doctor tells him to switch on both the inflow and the outflow pumps simultaneously. The result is a massive explosion in the Control centre which kills all the Macra. 

The colony celebrates its freedom. The Pilot decides that a dance competition should be held each year in honour of the travellers. Learning that the colonists want to make him their new Pilot, the Doctor decides that is time to return to the TARDIS.

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