London has a new tourist attraction. What used to be Hampstead Heath is now Space World, a Parakon Corporation theme park filled with games, rides, and supposed alien monsters from distant worlds. But few realise just how far Chairman Freeth and Vice-Chairman Tragan have travelled to attend the opening ceremonies. Two drunken passers-by, Bill and Nobby, break into the park the night before it opens to sneak a peek at the attractions, and are surprised to see a genuine spaceship landing. Nobby cheerfully rushes forward to greet the new arrivals, and Tragan, surprised, sets two vicious animals on the intruders, killing them instantly. As Freeth and Tragan’s horrified human associate Grebber watches, Freeth advises him to let the authorities discover one of the partially-eaten bodies. Any publicity is good publicity…

News of the peculiar death soon reaches UNIT, where the Doctor is trying to repair the TARDIS’ psychotelemetric circuit while his new friend Sarah tries to interview him. The Brigadier enlists the Doctor’s help in the investigation, and Sarah’s curiosity is piqued by his comment that the body appeared to have been savaged by a sabre-toothed Rottweiler. Sarah decides to attend Space World’s opening ceremonies, but her editor Clorinda isn’t enthused about the possible story, and instead of sending an experienced photographer she sends Jeremy Fitzoliver, an upper-class twit who barely knows which way to hold the camera. Before the ceremonies begin, the Doctor and the Brigadier pay a visit to Freeth, Tragan and Grebber, and the Doctor makes several innuendo-laden comments about aliens, hoping to provoke a response. Grebber is terrified, but is unsure whether to be more frightened of Tragan or of the Doctor. Nevertheless, Freeth realises that Grebber is becoming a liability…

Parakon’s spokesman Maroc Kitson guides the journalists and VIPs around the park but requests that they refrain from taking photographs of the “monsters from outer space”. The Doctor recognises some of the monsters as genuine aliens -- but also knows that they are not all that they seem. The tour continues on to the Experienced Reality grid, the showcase of the Corporation, and the Doctor asks the Brigadier to test the ER machine and see what happens. The Brigadier finds himself living out other people’s experiences, such as a day at the races or at the beach, but when the Doctor asks the Brigadier to interact with the programme he is unable to do so -- and finds that he doesn’t mind, as he’s happy enough to let the programme take him where it will. Meanwhile, Sarah and Jeremy slip away from the tour to take some surreptitious pictures of the alien monsters, but Grebber finds them doing so and asks them to take a message to the Doctor. Tragan catches him speaking with Sarah, and takes him back to the office suite, where he knocks Grebber out and plants an ER transceiver in his brain…

The Doctor’s and Grebber’s bodies are taken to the morgue, and the grieving Sarah returns to her magazine and tries to convince Clorinda to produce a feature on the Doctor’s life. But Clorinda isn’t interested in filling her magazine with fanciful stories about aliens and Atlantis, and soon Sarah has another mystery to investigate -- Jeremy returns from the photo lab, claiming that the photographs which Sarah took of the alien monsters have all come out blank.

The Brigadier tries to stop Freeth from opening Space World to the public until his investigation is complete, but his efforts are blocked at the highest levels of the British government and the United Nations by officials who claim that they are involved in delicate negotiations. The Brigadier then begins to wonder whether the Doctor’s remarkable constitution may have saved his life after all, and goes to the morgue to make sure -- just as the Doctor wakes up, badly startling the attending physician. As he gets dressed, he learns that Nobby’s mutilated body was brought to this same morgue, and takes the opportunity to examine it. The wounds were clearly made by no terrestrial animal, the saliva of the attacker appears to have been acidic, and when the Doctor studies the body more closely he finds a hair under the fingernail -- a hair which did not come from a mammal.

The Doctor and the Brigadier return to Space World to question Freeth further, and Freeth soon recovers from the shock of seeing the Doctor alive and plies them both with fine food and brandy. He admits that he and Tragan are indeed extra-terrestrial -- but they are here in peace, to offer the benefits of membership in the Parakon Corporation to Earth. The Doctor realises that the food and brandy is a blatant attempt to win them over, just as Space World is simply here to bring the Parakon Corporation into the public eye. Experienced Reality is just one of the great benefits they claim to offer the human race. There is still a death to be explained, but as the Doctor had already guessed, the “monsters” can’t possibly be responsible -- for they are only ER projections, and are not in fact there at all.

Now that he knows the Doctor to be alive, Freeth orders Tragan to return home with his “pets” to avoid any embarrassment. Meanwhile, Sarah and Jeremy return to Space World after hours to learn why their pictures did not turn out, and they soon discover the truth about the exhibits -- and spot Tragan entering one of the funfair’s spaceship rides, accompanied by two very real and vicious alien animals. Sarah follows him, telling Jeremy to contact the Brigadier and tell him what they’ve seen, but although Jeremy manages to locate a phone with little difficulty he is still trying to explain himself to the Brigadier when the “ride” suddenly takes off -- taking Tragan back to Parakon, and Sarah as well. Tragan’s pets soon sniff out the stowaway, and he decides that Sarah has become a liability who must be disposed of. He removes his humanoid mask to reveal his true, repulsive face, and ties Sarah up, telling her that it’s just as well she came along -- the trip to Parakon is subjectively long, and now he has a way to amuse himself on the journey…

The Doctor had foolishly assumed that the creature which killed Nobby was from Freeth’s home planet, but it appears that this is not the case after all. There seems to be no way to track down Sarah, until it occurs to the Doctor to wonder why the soldier was cursing the name of the Parakon Corporation so violently. Playing a hunch, the Doctor returns to the war zone and picks up some shrapnel from an artillery round -- and, as he had suspected, the ammunition turns out to have been manufactured on Parakon. The TARDIS thus materialises safely on Parakon, where the Doctor, the Brigadier and Jeremy contact the benevolent and aging President, introducing themselves as ambassadors from Earth. The President sends the captain of his personal guard, Waldo Rudley, to collect Sarah from Tragan’s home, while he speaks with the Earth delegates. He is appalled to hear about the war on Blestinu, which was once a happy and prosperous planet, and assures the Doctor and his friends that they were simply unlucky enough to arrive during a minor border skirmish, nothing serious at all…

Rudley arrives at “Fortress Tragan” to collect Sarah, and although Tragan at first refuses to let her go, Rudley -- who doesn’t care for the arrogant Tragan at all -- punches him in the stomach and takes Sarah away while the stunned Tragan struggles to recover. Furious, Tragan vows to have his revenge, and contacts Freeth to warn him that the Doctor and his friends are speaking with the President. Freeth orders Tragan to neutralise the Presidential guard in case they have to act quickly, an order which Tragan is all too happy to obey. The monitors at the Parakon Corporation look over Rudley’s records and find that he once made an off-hand remark which might possibly be interpreted as being somewhat critical of government policy. This is all the excuse Tragan needs to put him under surveillance. Sooner or later Rudley will slip up again, and they will have him…

Sarah is reunited with her friends, and when she describes the unmasked Tragan the Doctor recognises him as a Naglon, one of a race he’s encountered before. Waldo invites Sarah and Jeremy to his friend Greckle’s party while the Doctor and the Brigadier dine with the President, and when Sarah and Jeremy return to the guest quarters to prepare, they find an ER couch in the corner. Jeremy tries it out and is shocked to find a pornographic channel, and when Sarah tries the couch she finds a channel depicting what appears to be a paintball hunt -- until the man she shoots goes down with a scream and a spray of real blood. Waldo arrives to find her traumatised by the experience, but although he insists that the emotions she felt were simply transmitted from the real hunter, he is forced to admit that the hunt itself was real. As they travel to the party in a flying car, auto-piloted and locked into the downtown traffic grid, Waldo explains that the people of Parakon are idle and lazy, and spend all their days on ER couches living vicariously through others. The most popular channels carry the hunts and games, in which political prisoners and lower-class citizens risk their lives for freedom -- and the games are getting crueller and bloodier every day, while those who protest are accused of treason and find themselves fighting in the very same games they spoke out against.

The President tells the Doctor and the Brigadier about the wonderful plant rapine, which was first discovered by his grandfather. Everything on Parakon is manufactured or synthesised from rapine, which can produce anything from fuel to food to synthetic metal, and provides abundant harvests in any environment. The President himself was once a salesman for the Parakon Corporation, selling dreams of paradise to other worlds; and indeed, thanks to rapine, all the needs of the people are provided for. While the Doctor does not question the material benefits of rapine, he does wonder whether the people are spiritually fulfilled; but when he questions the President about the games, the President becomes uncomfortable and has to leave the room to rest. His caretaker and housekeeper, Onya, asks the Doctor not to upset him too much; he is a kind man and is loved by his people, but he is very old and unwell, and prefers not to face the harsh truths of modern life. She departs, knowing that she has said too much, and leaving the Doctor and the Brigadier puzzled by how well-spoken she seems for a bondservant.

Sarah, Jeremy and Waldo mingle with the shiny happy people at Greckle’s party, and Waldo shows Sarah that the party’s environment is an ER projection which can be altered to fit the mood of the partygoers. He becomes subdued when Greckle announces that later, they will watch the game in which the champion Jenhegger fights his latest opponent. Waldo’s old sparring partner Rasco Heldal questions his reaction, and they resume their old argument over the effect of the games on the people of Parakon. What neither of them realise is that Heldal was implanted with an ER transceiver the last time he was in hospital, and their entire conversation is being monitored by the listeners at the Parakon Corporation. As the argument escalates, Sarah sees the bloodlust rising in the eyes of the partygoers, and finally Waldo snaps that the games are corrupting society and that the government is only too happy to let it continue and gather up the profits. Corporation security guards immediately storm the party, arrest Waldo and drag him off. Sarah tries to protest, but Heldal and Greckle tell her to be quiet before she is arrested too. Sarah and Jeremy return to Waldo’s car and programme it to return to the palace, intending to inform the President.

The Doctor and his friends are imprisoned until Freeth can convince his father that they returned to Earth. Waldo is in a separate cell, having chosen to become a target of the hunt rather than participate in the sanctioned murder of the games. The next morning, Onya brings breakfast for the Doctor and his friends -- and knocks out one of the distracted guards. The Doctor deals with the second, and Onya leads them to a flycar which she had earlier separated from the traffic grid. Onya’s arm is paralysed by a shot from the guards’ stun-guns, but the Doctor is able to fly the skimmer to safety, and they are soon lost amidst the crowded traffic of the capital. Onya admits that she is working from within to bring down the evil Corporation. She used to be a scientist and to answer to the name Katyan Glessey, but she ran away from civilisation when she found she could no longer deny what her work was doing to her world and what the corporation was doing to her friends. She directs the Doctor to the Lackan, a mountainous terrain unsuitable for farming and thus the only surviving wilderness on Parakon. The capital city sits in the midst of an endless desert; rapine has sucked all of the life out of the land.

In the middle of the Lackan is the home of the last free tribe of Parakon, the Kimonya. Katyan lived with the Kimonya after fleeing from the city, and with the help of a wise old teacher, she learned to heal her emotional scars and took on the name “Onya Farjen”, which means “Sky Born”. When her teacher passed on, she became the tribe’s new teacher and healer, but she knows that she still has much to learn, and she therefore returned to the city to help others escape and plot to overthrow the corporation which is destroying their world. Onya lands some distance away from the Kimonya as a precaution, and operates a deactivator just in case any of the others were implanted without their knowledge. The Doctor heals her paralysed arm, and she leads them through the Lackan to her home. Despite the dangerous animals and plants, the worst obstacle they face is Jeremy’s constant complaining, and they are eventually forced to stop to give him a rest. This turns out to be fortunate, as a monstrous Gargan then lurches past, and Onya realises that she nearly led them right into its territory without noticing; if they’d set one foot across the border of rocks the Gargan had laid out, it would have taken their scent and would never have stopped hunting them.

Somehow, the Kimonya know that their adopted sky-mother is on her way, and much to the Brigadier’s surprise the tribe flies out to greet them on the backs of tame giant bats. In the village, Onya introduces her new friends to their fellow refugees; Rance is working to transform the Corporation’s paralysing weaponry into less harmful stun guns, while Medan monitors ER transmissions in the hope of rescuing victims of the hunt. He manages to locate Waldo Rudley just as Rudley, who never watches the games and is thus unaware of the dangers, tries to shelter from the hunters in the Gargan’s territory. One of the hunters shoots him in the back while trying to wing him, and the wounded Waldo crawls over the Gargan’s border. The hunt is now effectively over, and the hunters depart, knowing that Waldo’s as good as dead. Freeth, who has been watching in the city, is disappointed, the hunt was over far too soon, and since Waldo has passed out from the pain of his wound, his ER implants are not transmitting and Freeth will miss out on his death.

The Doctor and the Brigadier must cross into the Gargan’s territory themselves to rescue Sarah and Jeremy. The Doctor tries singing the attacker to sleep with his Venusian lullaby, but the Brigadier’s stun-gun proves more effective. The Doctor, however, knows that if the Gargan is dead then they’ll have lost the aid of the Kimonya -- and if it isn’t then it will hunt them until they’re dead. Nevertheless, he and the others take Waldo back to the tribe, who are holding a great feast to celebrate Onya’s return. Onya tries to tend to Waldo, but fears that he’s lost too much blood to live through the night. The Doctor tries to convince Sarah to join the others, as there’s nothing she can do for Waldo, but the feast is interrupted when the Gargan emerges from the forest, still pursuing its prey. The Doctor wraps up pieces of meat in rags stripped from his friends’ clothing, and has the Brigadier fling them at the Gargan -- which eats the meat and departs, convinced that it has devoured its prey.

The Doctor’s quick thinking has won the respect of the Kimonya -- but in the meantime, Waldo has died of shock and blood loss. Worse, Onya realises that she’d forgotten to deactivate his ER implants. Although Waldo never regained full consciousness, the corporation may still have been able to trace the location of the Kimonya tribe. They have no choice but to go on the offensive before they are attacked. Rance admits that they have enough weapons to overcome their enemies, but they have no way of getting to the city -- until the Doctor suggests using the giant bats. The Kimonya are only too happy to help the Doctor and Onya, and the Brigadier prepares to draw up a plan of attack. By the time the Corporation’s security guards realise that the flock of bats approaching the city is not an ordinary migration, the attack will be underway.

While the Brigadier organises the attack, the Doctor, Sarah and Onya return to the city in Onya’s flycar, hoping to get the President on their side. The Doctor first has Onya take him to the data storage library, believing that he will find evidence of a far greater crime than anything they’ve seen so far. Onya still has the ID which identifies her as Katyan Glessey, and with this authorisation the Doctor accesses news reports and video files. As he had suspected, rapine farming has glutted the natural resources of the first worlds to join the Parakon Corporation, and social unrest is building as their economies break down. But there is worse to come. The war which has broken out on Blestinu is not being fought with biological or chemical weapons... those might damage the bodies. Civilian refugees are being taken to processing chambers and gassed, and the bodies, both of soldiers and of refugees, are being ground into fertiliser for the next world down the line.

Onya takes this news straight to the President, but the Doctor and Sarah remain to collect more evidence -- only to be captured by Tragan and his guards. Tragan brings his prisoners to Freeth, who decides to humiliate the Doctor before his death by forcing him to participate in the games. The Doctor will be forced to dress as a clown and must fight the champion Jenhegger armed with nothing more than a rolling pin. If he refuses to do so then Sarah will be killed slowly. The Doctor has no choice but to agree. Meanwhile, Onya gets to the President, who is delighted to see her again -- until she breaks the terrible news of what his dream has become, and what his son has been doing in his name.

The Brigadier and his men storm the city, and just as the Brigadier had predicted, the security guards assume the bats to be migrating until they get close enough for the guards to see the humanoid figures on their backs. The bats prove to be far more manoeuvrable than the Corporation flycars, and the Brigadier and the rebels land at the games arena headquarters -- where most of the guards are watching the games on ER, and fail to notice the rebels entering the security HQ until it’s too late. The Brigadier rescues Sarah, and places the fuming Tragan under arrest.