A Macra

On an Earth colony in the far future the feel of a holiday camp is shattered when a man, called Medok, is seen to be on the run and one of the leaders of their planet's seemingly idyllic colony - the Pilot - orders Ola, the Chief of Police, to send guards to recapture him. But what is he running from and why are the authorities so keen that he is recaptured?

Meanwhile, nearby the TARDIS materialises and The Doctor and his three companions, Polly Wright, Ben Jackson and Jamie McCrimmon disembark cautiously. They have just seen on the scanner a clawed creature and so are expecting trouble. However, they are not prepared for Medok to crash into them. Ben is able to hold onto him until guards arrive. While Medok is dragged off, Ola thanks The Doctor and his friends for helping to catch Medok – who he calls a dangerous criminal. However, as Medok is escorted away he screams back that he knows the secret behind the colony.

The Doctor and his companions are invited into the colony to receive the personal thanks of the Pilot. But instead of a real person a still picture of the colony's Controller appears on a screen and through speakers they hear his voice thanking them again and orders everyone in the colony to enjoy themselves. The travellers are then invited to relax and to sample the many healthy therapies available.

Everyone is Happy

The Doctor though is curious about Medok’s outburst and so is not convinced that everything in the colony is at it seems. His suspicions are proved to be correct when he sneaks off to locate Medok, who he finds in the rehabilitation centre, and is told about the crab-like creatures that move up to the surface at night. Medok though is terrified and before he reveals any more he make his escape from the rehabilitation centre.

The travellers then learn of a curfew that is operated during the hours of darkness and are ordered to go to rooms that have been prepared for them. Despite the curfew The Doctor sneaks out of the colony and comes across Medok again. However, guards spot them and as they close in The Doctor and Medok flee. It is then that one of the creatures appears in their path. Medok tries to show the creature to a guard, but is surprised when he refuses to acknowledge its existence. Instead The Doctor and Medok find themselves arrested and dragged off to face the Pilot. Thanks to Medok, who is returned to the rehabilitation centre, The Doctor is freed.

Meeting the Cononists

Meanwhile in their quarters Ben, Jamie and Polly are sleeping – after being put into a hypnotic state by hidden machines. A voice is heard telling them to follow the rules of the colony, not to question anything and to work hard. In the men’s quarters Ben is in a deep sleep but Jamie is restless.

Early the next morning The Doctor discovers that while Polly was asleep someone had tried to hypnotise her. Luckily Polly seems to be unaffected. However, when they rush to their friends' room they find Jamie is also unaffected but Ben it seems has succumbed. The Doctor destroys the hypnosis machine. But this enrages Ben who rushes off in disgust to report his friends to the guards. It soon becomes clear that Ben has come under a malign influence of whoever is really controlling the colony and this has made him turn against his friends.

It is therefore up to The Doctor, with the help of Polly and Jamie, to discover the truth as to what is hiding in the depths of the colony. They discover that the supposedly idyllic colony has in fact been infiltrated and taken over by a race of giant crab-like creatures - The Macra – and that the brainwashed inhabitants are forced to mine a gas that is toxic to themselves but which is vital for their oppressors' survival.

Medok is Captured

Even when Polly, who enters the underground mines in search of The Doctor, comes under attack from a Macra. Ben, who has followed her, does not at first come to her rescue. It seems that his conditioning prevents him from even acknowledging The Macra’s existence. However, as more creatures appear, surrounding them both, the sight of Polly in danger temporarily breaks Ben's conditioning and he rescues her. But as soon as they are safe his conditioning re-asserts itself. It seems that there is nothing that can be done to break The Macra’s influence over all those in the colony.

With Polly and Jamie forced to work in the mines, and so in danger of dying from the toxic atmosphere, they learn that The Macra are totally reliant on the gas to exist. With this knowledge they try to escape. In doing so Jamie enters a forbidden area and is nearly suffocated when more gas is pumped into the room. Luckily The Doctor has also learnt of the importance of the gas and when he discovers Jamie’s predicament he hastily shuts down the pipe that is pumping the gas. He then manages to reverse the flow, pumping fresh air down into the tunnels. The Doctor and Polly then join Jamie in the forbidden area so as to confront The Macra. But they soon find themselves in danger when the voice of Control orders the gas to be restored to the tunnels. Jamie is able to escape to the surface but is quickly captured by the guards. The Doctor and Polly find their way to the Macra-infested Control Centre deep in the tunnels. But when they manage to return to the surface, and then persuade the Pilot to come and see what they have discovered, The Doctor and Polly are captured by Ola, who The Macra have put in charge of the colony when their control over the Pilot is lost.

The Doctor, Polly and Jamie are in danger of being gassed to death in the pump room and it seems that the Macra are unstoppable. However, Ben’s conditioning is starting to wear off and when he regains enough of his senses he is able, under the guidance of The Doctor to manipulate the gas pipes to send a combustible mixture of gas to the Macra’s Control Centre. When it explodes all The Macra are destroyed - thus saving Ben’s fellow travelling companions and restoring the colony’s freedom.