The planet Nooma is enclosed between the ground and the living Sky. A man named Epreto leads an expedition to another Land to find a fallen Sun, but while there, two of the expedition members succumb to the dictates of their biology and fight to the death. In the course of their struggle the Sun is partially reactivated, and Epreto learns all he needs to know to carry out his great plan. Upon returning to his Land, however, rumours of his true intentions leak out and soon the Land and the Sky are in upheaval. Epreto's friend Lofanu, fearing that Epreto will destroy their world, realizes he must betray him, but Epreto is aware of his intentions. While travelling to the place of the Dead, Epreto has his men take Lofanu and tie him up in the childforest, leaving him to the savage, carnivorous children...
Sergeants Benton and Bell set up Mike Yates and Jo Grant on a blind date with each other as a joke. They decide to invite the Doctor out to dinner with them, but he offers to take them both to dinner on the planet Karfel. Mike, intrigued, accepts the offer, but the TARDIS goes off-course and ends up on Nooma in the childforest. They see Lofanu tied up and attacked by savage winged creatures. The Doctor heads out to help, and Mike follows, but Mike, unprepared to find himself on top of a tree on a planet where the gravity is only one-sixth that of Earth's, tumbles over the side and falls. The Doctor is forced to follow, using a pair of inflatable wings to glide to safety, and once on the ground they find spent bullet cartridges, indicating that this is not a safe place to be.
Jo emerges from the TARDIS too late to save Lofanu, but a man who appears to be made of wood appears, telling Jo that he is Dead. She agrees to help him bury Lofanu so he too may join the Dead, and then asks Jobanu to help find her friends. He takes her to a place where she will find all the answers she needs, but once there she is sucked into a tree which changes the way she thinks. She is told to bring the Doctor to the Dead -- he has knowledge they require.
The Doctor and Mike climb the tree to find the TARDIS missing, and Epreto's airship then returns from its meeting with the Dead and finds them. Epreto tells them that their friend and ship will have been taken by the Dead, and agrees to help, but Mike suspects that his intentions are not honourable. In fact, Epreto has recognized the Doctor's intelligence, and hopes he will help Epreto to free his people. Mike escapes before the airship lands at Epreto's home, and follows it at a distance, hoping to rescue the Doctor. The Doctor, meanwhile, learns about the biology of the Noomans. Children bud in the forest, grow to maturity and become men; men fight to the death, and the winner rips out the loser's heart and eats it. The winner is then Promoted and becomes a naieen, a gendered creature capable of flight; the loser is returned to the Land and becomes one of the Dead. When naieen mate, their seed falls on the forest, fertilising it.
Epreto hates the trap of biology, feeling that only men -- preyed upon by savage children and subservient to the complacent naieen -- are capable of real progress. Furthermore, he has determined that the Suns of the five other Lands have all died, and has concluded that their own Sun will one day die as well. He claims to be trying to reactivate the dead Sun, to take his people away from this world before that happens. Before agreeing to help, the Doctor consults the legends which Epreto had dismissed as superstition, and determines that the Noomans are likely the results of genetic tampering; as such, their biology is linked to this world, and if they leave they will die. The Doctor offers to help Epreto find some way to extend the Sun's natural life span, but Epreto doesn't accept the Doctor's claims and decides not to tell him the truth -- that he in fact intends to steal the living Sun of his own Land.
The naieen have learned of Epreto's true intentions and are massing an army with which to stop Epreto from changing the natural order of life. The winner of the fight in the other Land is now a female naieen named Xaai, and she is held captive and tortured by the other naieen until she remembers the expedition and tells them about Epreto's plan to steal the Sun. There have been other attempts to circumvent Holy Biology, and one man in particular, Karilee, is summoned to the naieen. He has been experimenting with extracts of heartmeat to find a way of artificially ensuring Promotion, and although the naieen have no intention of letting him succeed, the elderly Aapurian promises him help if he finds out what Epreto is up to. Meanwhile, Epreto sends word to one of his associates, Duboli, about the new "weaker" men he has found in the childforests, but the message is accidentally intercepted by a man named Omonu. Omonu is weak and fears dying in the final fight of his life, but if he fights one who is weaker than himself, he is sure to win and be Promoted...
Karilee finds Jo near Epreto's home, speaking of the childforest and the Dead, and decides that she may know something of Epreto's plans. He takes her with him to Epreto's, where they find Mike searching for a way in. They try to break in through the cellars, but find that Epreto has trapped an Unpromoted man in the cellars, disfigured and insane as his biology urges him to fight someone who isn't there. Mike is startled when Jo urges him to let the Unpromoted man fight and kill him, but then Omonu arrives, creating a distraction which enables Mike to shoot the Unpromoted. Omonu offers to help them search for a way up into the house, but manages to get Mike alone, attacks him and rips out his heart. Karilee catches Omonu and, disgusted by his cowardice, shoots him, but Jo is shocked out of the conditioning imposed upon her by the Dead when she realizes that Mike is dead. Karilee buries Mike, but by the time he and Jo enter Epreto's home they find that he has already departed with the Doctor.
The Doctor suspects that due to Epreto's hatred of the naieen's complacency he hasn't done all he can to persuade them of the danger. As they approach the Sun, they find an army of naieen blocking their approach. Epreto's men open fire on the naieen with machine guns, and when the Doctor tries to stop them he misjudges his jump and falls over the side of the steamwing, landing amongst the naieen. The naieen army's leader, Eeneeri, senses that he is a reasonable man, and agrees to take him to the naieen temples to speak with the Flight and learn their side of the story. Epreto, meanwhile, fights his way through the naieen army but is then shot at by a naieen, using a "magic" gun capable of locating an opponent's heat signature. Epreto deliberately pilots the steamwing to the wrong entrance to the Sun, which perceives it as an intruder and shoots it down. He has already leapt from the steamwing, and the flare of its explosion exposes the naieen, whom Epreto kills. He then enters the Sun by reciting the secret password, "Aapex".
Karilee overpowers Epreto's guards and finds his secret notes, which reveal his true plans to use a Landkilling poison acquired from the Dead to destroy the Sky, breaking it open so the Sun can pass through. Duboli arrives and releases the guards, who kill Karilee; Jo escapes but is pursued, and steals a pedithopter, trying to seek sanctuary with the Flight and warn them of the danger. Meanwhile, Xaai awakens and sees the bodies of naieen killed in the battle with Epreto's men. She blames herself for the carnage and flings herself from the gondola, but collides with Jo's pedithopter. When they compare notes they conclude that Epreto intends to administer the poison at the naieen's temples, and they attempt to reach the temples first to warn the naieen of the danger.
The Doctor manages to convince Aapurian that the biology of the Noomans is the result of an illegal genetics experiment conducted five thousand years ago by the Aapex Corporation. He builds a device which should enable him to shut down the Sun by remote control, but the Sun detects the intrusion and informs Epreto of the danger. Epreto thus turns the Sun's weapons on the naieen temples, and while the Doctor and Eeneeri escape, Aapurian remains to die with his temple. The Doctor goes to the Sun to reason with Epreto, but Epreto is convinced that he is leading his people into a glorious future and won't accept the Doctor's claim that the Sun's database is five millennia out of date and that the verdant green worlds he sees have long since been colonised by other people. When the Doctor tries to shut down the Sun's navigational programme, Epreto tries to kill him, and in the struggle the Doctor falls out of the Sun without his gliding wings.
Jo and Xaai are caught on the edge of the explosion when Epreto destroys the temples, and Xaai falls to the ground, where she is captured by an army of the Dead led by Mike Yates. Epreto had told the Dead that he required the poison to fight an attack by another Land, but they have realized that he lied to them. When Xaai reveals Epreto's true plans, the Dead give her an antidote and send her to the Sky. Epreto, certain that the naieen have discovered his plans, decides to move ahead of schedule, and after a brief struggle with his conscience Duboli poisons the Sky. Epreto shoots Xaai down before she can administer the antidote, but Duboli, finally won over by the Doctor's claims and by the growing evidence of Epreto's obsession, tries to fight Epreto to stop him from destroying the world. He hasn't realized how close Epreto is to promotion, however, and the fight turns into a duel. Epreto wins, ripping out Duboli's heart, eating it and cocooning himself as he begins to transform into a naieen.
Jo is rescued by the Sky, a sentient organism which questions her about the explosions. She warns it about the poison, but the infection is already too advanced, and it gives Jo a travel pod with which she can fetch an antidote from the Dead. On the way down, the pod encounters the falling Doctor and rescues him, and Jo takes him to the Dead, who try to alter his mind with the same pseudotelepathic viruses they used on Jo. He is able to resist them, agreeing to help them only on his own terms. Fearing that the TARDIS is too unreliable a means of transportation, he hopes to use the travel pod, but they are attacked by the carnivorous children and delayed for too long. The pod is already dying, and the Sky has burst into flame; fragments of the burning Sky are falling on the Land below, devastating it.
The Doctor uses the TARDIS to take the Dead to the Sun to conduct repairs, but once there, the Dead begin to carry out the emergency instructions of the Aapex Corporation. Since word of the illegal experiment cannot be permitted to get out, the experiment must be aborted, and the Dead thus begin to slaughter Epreto's followers. Jo tries to stop Mike, who shoots her. The Doctor lies to Mike, claiming that she's now brain-dead, and Mike hesitates, confused. This gives the Doctor the opportunity he needs to access the Sun's data banks and prove that the emergency instructions have changed; the Aapex Corporation terminated the experiment over two thousand years ago and granted all of Nooma's inhabitants legal status as sentient beings. The Dead stand down and allow the Doctor to begin repairs.
After much hard work, the Sky is restored, the Sun is put back in its place, and even Mike is restored to life in his original body. He has no memory of anything that happened after his death, and Jo decides not to tell him that he shot her in cold blood. Epreto, now a naieen, has learned his lesson, and agrees to work more slowly towards change on his world.
Source: Cameron Dixon