For 20 weeks, Britain has been affected by a drought and the Government is forced to announce a national state of emergency following a number of recent public disturbances. From now on, people will be prosecuted if they’re found to be exceeding their water standpipe rations. A demonstration is taking place in London, attended by 100,000 protestors calling for increased efforts to combat global warming. The event has been organised by a pressure group called the League of Nature, led by former rock star, charity worker and now environmental activist, Alex Marlow. Recently he’s adopted a more militant stance, claiming that the situation the world is facing is becoming increasingly extreme. Several species have already become extinct in the last few years and his group is proposing compulsory population reduction, the abolition of private capital and any other action they deem necessary to save the world. Some commentators believe his campaign is starting to become threatening, but he argues that if people don’t take action now, all life on Earth will be endangered. Another member of the League of Nature is a young Northern girl called Lucie Miller, who admits that in order to save the world, their group is prepared to resort to intimidation and violence. Not only is she ready to die for her cause, she’s even willing to kill for it…

Two weeks later and there’s still been no rain. Reservoir levels have decreased to such a degree that water supplies are now restricted only to hospitals.

The Hothouse is a top secret research facility owned by Alex Marlow, which operates under a complete security blackout. It lies in a heavily guarded, fenced-off compound covering over 10 square kilometres and contains five state-of-the-art biodome greenhouses, like a private Eden Project. The League of Nature’s profile has increased dramatically in recent years and it now has over 80 million paid-up members, but all the money seems to have been pumped directly into the Hothouse. All the scientists working there are associated with GM tech research and the World Ecology Bureau is worried about what he might be doing there. As a result, the Bureau has contacted UNIT and asked them to summon the Doctor using the space-time telegraph. Their representative, Hazel Bright, briefs the Doctor as they head to the compound in a chauffeur driven car, but she’s surprised when he tells her he’s pleased that someone is finally taking the future of the planet seriously. In fact, he believes most people are complacent and never take responsibility for their actions and he‘s beginning to wonder what he ever saw in the human race.

Hazel and the Doctor arrive at the Hothouse and announce that they’ve come to conduct a routine random inspection under the St Petersburg Bio-Protection Treaty. They’re greeted by Christina Ondrak, Head of Administration, who assures them their visit won’t be a problem but says they must be accompanied at all times for their own safety. She leaves to arrange their passes and hands them over to her assistant, Miss Lucie Miller. The Doctor says he remembers seeing Lucie from a news report on the protest demonstration and asks her why they need to have so many guards here, but Lucie tells him she doesn’t have authority to discuss such matters. Realising their every move is being monitored by CCTV camera, the Doctor ’accidentally’ spills his tea over Lucie, causing her to drop her security pass. He hands it back to her just as Christina arrives with the new visitors passes. Watching from his office, Alex Marlow realises straight away that this must be the famous Doctor he’s heard so much about…

As part of their tour, the Doctor and Hazel are taken to Biodome 1 and pass through an airlock which maintains the atmospheric integrity. Inside is a jungle containing many plants species that no longer exist anywhere else on the planet. Christina says they’re conducting research to discover whether the plants might have therapeutic uses or could be cultivated as a source of nutrition, but their main work is concerned with conservation and they hope to use to seed new rainforests across the world. The Doctor recognises the scent of ozone in the air and Christina says they maintain an environment here that will encourage the plants to grow. The Doctor and Hazel ask to see the other Biodomes, but Christina tells them access is restricted as some of the species there are in a very vulnerable condition. As she takes them back to reception, the Doctor asks if they can meet the owner, Alex Marlow, but Christina says he’s too busy and they’ll have to arrange an appointment later. Leaving the compound, the Doctor and Hazel are frustrated. They were shown just enough to make them go away, but not enough to answer their questions. What they really need is someone on the inside.

At the reception desk, Lucie tells Christina she needs to finish off a few emails before she goes home. Christina says goodbye, but as soon as she’s alone, Lucie examines the security pass the Doctor managed to slip her earlier. Unlike the one provided by her employers, this one has been adapted to give her access to all areas. She switches on her hidden microphone and contacts the Doctor who’s back in the chauffeur driven car with Hazel. The Doctor assures Hazel he trusts Lucie and says she volunteered for the assignment as she was keen to prove herself to him again.

Lucie arrives at the airlock for Biodome 4, which she’d discovered is the one they’re most desperate to keep secret. She uses her pass to enter the dome and checks that she’s alone. Inside, she finds a huge laboratory filled with green coconut-sized pods under lamps set up to encourage their growth. When she describes them to the Doctor he orders her to get out immediately - but before she can leave, Marlow steps out from cover and points a pistol at her head. He tells her how disappointed he is that she’s betrayed him. He admits that she had him completely fooled and says it’s a shame she’s not a real believer of his cause. Using Lucie’s own microphone, he says hello to the Doctor and Hazel, then tells them he can’t let Lucie go as she’s already seen too much. Unfortunately this means he’s going to have to sacrifice her for a higher purpose.

In their car, the Doctor and Hazel listen in horror as Lucie screams, then the microphone goes dead. The Doctor is furious with himself and orders the chauffeur to turn round and head back for the Hothouse. He tells Hazel he now knows the true nature of Marlow’s research - what Lucie described were Krynoid pods. From the Bureau files, Hazel is already aware that on worlds where the Krynoids take root, the vegetation fights back and if just one of them manages to germinate, every human being and animal on this planet will be dead within a matter of hours. The Doctor wonders how Marlow got hold of the pods and Hazel is forced to admit that after the Harrison Chase incident, Sir Colin Thackeray ordered that cuttings be taken from the Krynoid remains so they could be studied to find new ways to kill them. About five years ago there was a break-in at the Bureau and the cuttings were stolen from their vaults. The Doctor says Marlow doesn’t know what he’s dealing with, but rather than drive up to the front door, he tells the chauffeur to pull over, then he and Hazel prepare to break-in unnoticed through the perimeter fence.

The compound is surrounded by a fence about 12-feet high. The Doctor realises it’s about time he re-designed his sonic screwdriver so that it can cut through wire, but for now he and Hazel have nothing that will do the job. In any case, the area between the fence and the buildings are patrolled by armed guards. They hear vehicles approaching and watch as container trucks arrive, presumably filled with supplies. The lorries stop at the gatehouse and the Doctor leads Hazel to the back of the nearest one. As they climb up, they’re shocked to discover they’re not the only people inside…

In his office, Marlow watches in amusement as his adversaries smuggle themselves inside the facility like common criminals. He turns to Lucie and tells her it’s touching that the Doctor thinks he’s coming to rescue her…

In the back of the lorry, Hazel is horrified to see that their fellow travellers are half-starved and frightened to death. She tries to assure them they don’t mean any harm, but the Doctor tells her not to bother and suggests they get ready to jump out at the first opportunity. These people are obviously being brought here for a reason, but they won’t find out what it is if they get themselves captured too. He and Hazel jump from the back of the truck and take cover near some storage tanks. They watch as the lorries come to a halt and guards start removing hundreds of people, like cattle being taken to the slaughter. From their appearance, Hazel concludes that they must be illegal immigrants, smuggled into the country from outside the Eastern Eurozone. She explains that last year, when the refugee crisis first hit, millions of people tried to get out of North Africa and the Black Sea states, so a decision was made to shut down the borders to avoid the Eurozone being overwhelmed. The Doctor is disgusted and says he’d forgotten how humans always seem to make the wrong decisions.

Marlow watches as the Doctor and Hazel follow the immigrants inside, then sneak up some stairs and through a door that’s been left conveniently unlocked. They disappear into an access tunnel and Lucie wonders why Marlow isn’t alerting his guards to stop them. Marlow tells her it’ll be much more entertaining to let them sneak in, believing they’ve remained undetected. They watch as the Doctor and Hazel head for Biodome 4, then Marlow says it’s time he made his move…

The Doctor and Hazel arrive in the laboratory. They examine the Krynoid pods housed inside the hydroponic incubators and the Doctor notices that their texture and shape are different to the ones he encountered before. They explore deeper into the dome and discover more than 50 people chained up and locked inside cages in various stages of metamorphosis into Krynoids. This is obviously what happens to the immigrants being brought here. Hazel is nearly sick and leaps back in shock as one of the human-Krynoid creatures leaps towards her from inside its cage. The Doctor explains to her how the process works and how human flesh is transformed into vegetable matter. One of the creatures tells them he used to be Stefan Radek, a school teacher from Poland who was lured here with the promise of work. He left behind a wife and two daughters, but he says it’s getting increasingly difficult to remember them. He recalls being brought here and given injections, but the Doctor is concerned because the rate of infection is different to what happened last time. Hazel is shocked when he admits that he can’t cure these people. Stefan has seen the others become monsters and he begins to panic, pleading with them to kill him, but the Doctor says there’s nothing he can do for them. Stefan starts screaming in pain and then his body transforms into the full Krynoid form. He starts to break free from his cage - and then suddenly a shot rings out and he falls to the floor, injured but not dead.

The Doctor promises to do whatever Marlow wants so long as Lucie is spared, so Christina is told to remove her from the cage. However, Marlow says he still needs a guinea-pig, a redundant member of the human race who is surplus to requirements. He suggests Hazel might be a suitable candidate. Hazel protests, but Christina grabs her and forces her over to the opening seed pod. Hazel’s screams come to a sudden stop when the tendril wraps itself around her bare arm and then Christina releases her. Hazel collapses and Marlow orders his guards to take her to the observation room. He also orders Lucie to be locked in with her as an insurance policy to guarantee the Doctor’s co-operation.

Marlow tells the Doctor he’s been researching the Krynoid species and knows it’s a parasitic infection that possesses the host mind before developing into a creature capable of communing with other plants. The Doctor warns that if just one of the experiments escapes, it’ll mean the end of all animal life on this planet, but Marlow says he’s well aware of the dangers. His plan is not to destroy the world, but save it. If he can create a genetically engineered variety of the Krynoid, he can then use it as nature’s own army. Unfortunately the Krynoid instinct to kill always becomes dominant, so he needs to find a way to make sure they always stay under human control. The Doctor accuses him of kidnapping, torturing and murdering innocent human beings, but Marlow argues that it’s an acceptable alternative to the destruction of Earth. Marlow says he sleeps easy in his bed, confident that one day people will look back and say he did the right thing. He believes the current world population isn’t viable and children are being born into a future where they’ll die of thirst with no hope. He thinks humanity needs to be pruned back to a sustainable level, but the Doctor says he’s reducing genocide to the level of horticulture. Marlow plans to implement a programme of mass sterilisation to bring the population down to 100 million individuals and asks the Doctor if he’s willing to help him. The Doctor admits that he’s been away for a long time and says that when he looks at the human race now all he sees is a species hell-bent on its own extinction that doesn‘t deserve to survive. But that’s exactly why he’s not going to help Marlow.

Lucie bangs on the door of the observation room and demands to be let out, but there’s no reply. Hazel is starting to come round and realises with horror that the skin on her arm has transformed into some kind of congealed sap. She realises she’s going to suffer the same fate as the poor victims in the laboratory. Lucie tries to get her to stay calm and assures her the Doctor will help, but Hazel screams at her furiously, knowing full well there’s nothing he can do. She can’t believe she was stupid enough to become involved with Marlow and promises Lucie she didn’t know anything about what he was doing here. She says she joined the League of Nature to stop the destruction of the planet after witnessing 10,000 bodies floating down the Ganges during a flood. She can actually feel the infection spreading through her body and warns Lucie not to get too close. She starts to hear the thoughts of the rainforest and can feel it breathing and growing inside her head. Her face is beginning to turn green and then her voice takes on the rattle of the Krynoid. She starts describing Lucie as “delicious animal flesh” and says she looks forward to consuming her…

In his office, Marlow watches what’s happening on a CCTV monitor. The Doctor insists that he let Lucie out, but he refuses unless he agrees to help. They notice Hazel is beginning to influence the plants inside the Hothouse, but Marlow remains confident they won’t come to any harm. The Doctor realises the effect is happening too quickly and the transformation shouldn’t have reached this stage yet and Marlow says it’s a side-effect of the new strain they’re testing. The Doctor is horrified - thanks to their genetic tampering, the process has been massively accelerated and if it continues at its current rate, the Krynoid will achieve its final form in just over an hour.

Lucie urges Hazel to fight the effects, but she’s becoming deliriously hungry. She says she can reach out with her mind to every plant in the building and hear what they hear. Through the plants in Marlow’s office, she can hear the Doctor explain that it might take months to find a solution to their problem. Marlow needs him to find a way of preserving the vestigial human consciousness so the Krynoids remain under their control. The Doctor insists that the human mind isn’t strong enough and all their research has been for nothing so they might as well end the experiment now. Just then, Christina contacts Marlow to report that the Krynoids in Biodome 4 have gone berserk and are smashing themselves against their cages. As she speaks, the creatures start twisting the metal bars and within seconds they’ve released themselves. Christina orders the guards to kill them and the air is filled with the sound of gunfire. It has no effect and the Doctor calls over the intercom for everyone to evacuate the area immediately. They hear Christina scream and then the intercom goes dead. The Doctor tells Marlow they’ve lost - he hasn’t saved the world at all, he’s betrayed it. He asks Marlow to release Lucie, but it already may be too late. There are dozens of Krynoids loose in the building and they’re heading straight for the observation room…

A near-fully transformed Hazel welcomes her Krynoid ‘brothers’ and summons them closer so they might set her free. For a moment, she reasserts herself and warns Lucie she can’t resist the effects for much longer and Lucie tells her to hold on to a treasured memory, perhaps of someone she loves, but it’s no good and Hazel remembers the Doctor saying the Krynoids always win in the end. Lucie tries a different approach and tells her to think about how much she hates Marlow for what he’s done and hold on to the human emotion of revenge. The Krynoids arrive outside the observation room and start banging on the door. Suddenly a loud klaxon sounds throughout the building. Hazel gains control again, but she’s not sure how long she can hold the Krynoid off. She tells Lucie to go and, to Lucie’s amazement, the other Krynoids step back and let her pass.

Marlow and the Doctor are watching in amazement. The Doctor guesses that Hazel herself is exerting an influence over the other Krynoids to give Lucie time to escape, which means she must still be in there somewhere. Marlow is delighted as this means his experiments to allow the human host to remain dominant have worked after all. To their surprise, Christina, who they assumed was dead, arrives in the office and warns that the Krynoids are everywhere. The guards are either hiding or are dead. Over the monitor they hear the Hazel-Krynoid creature calling Marlow’s name and Christina suggests they might try giving them what they want. She pulls a gun on Marlow, but the Doctor tells her she can’t bargain with the Krynoids. While she’s distracted, Marlow pushes her aside and flees from the room, deadlocking it behind him. Unfortunately the door may be strong enough to keep the Doctor and Christina in, but it won’t keep the Krynoids out for very long. Bullets have no effect on them, but the Doctor points out that anything that can kill plants will kill Krynoids too. Christina says they keep a supply of herbicides in the equipment store, but first they have to get there and the Krynoids are massing in the corridor outside. Lucie is hiding up on the roof of one of the greenhouses and when the Doctor discovers her hidden microphone is still working, he calls her for help.

Marlow flees outside the compound and summons a car to collect him. Just then, the Hazel-Krynoid tracks him down. He tries to convince the creature that they’re both on the same side and promises to help it. He offers to provide it with a super-enriched bio-feed and says there are powerful lights in Biodome 1 that will help it grow, then he asks the Krynoid for its response…

The door to Marlow’s office suddenly breaks open and the Krynoids start to come through - but then there’s a burst of freezing gas and they begin retreating. Through the smoke, Lucie appears carrying a fire extinguisher and points out that the Krynoids don’t seem to like the cold. The Doctor is delighted and wonders how he could ever have forgotten how brilliant she is. Christina says they won’t be able to hold the creatures back forever, so they head straight for the equipment store to see what it has to offer. They remove all the shredders and trimmers until eventually Christina emerges with the herbicide sprays. The Doctor is sure they can use these to fend the Krynoids off for a while, but he knows it’ll take more than this to defeat a Krynoid that’s achieved its final state. Christina says they only need to get to safety, but the Doctor knows no one else is going to come along to sort the problem out. They’re the last hope for this planet and as soon as one of the creatures reaches the primary point of germination it’ll release hundreds of spores, which will mean the end of the human race. Christina isn’t interested and says it’s no longer her responsibility. She tries to defend her previous actions and argues that she was just doing what she had to do to avoid starving to death back home.

The Doctor has heard enough and tells Lucie to collect as much weed killer as she can, but as they leave the equipment store they find themselves face-to-face with Marlow and his new friends - the Krynoids. The man is now clearly insane and he sends the Krynoids into the store, where he knows Christina is hiding, so they can feed. Christina tries to fend them off with a trimmer, but one of the creatures snatches it from her grasp and uses it against her. Lucie turns away as Christina screams… While Marlow is distracted, the Doctor and Lucie start spraying the herbicide directly at the Krynoids, then they run for their lives. Marlow calls after them and says they’re wasting their time as there’s nowhere for them to hide.

The Doctor tells Lucie they must reach Biodome 1 as they have to get to the Hazel-Krynoid before it releases its spores. Using the specially adapted pass card, they enter the dome and find the entire jungle is moving of its own accord. A branch reaches out and grabs Lucie’s hair, but as she struggles free the Doctor warns her not to hurt its feelings as the Krynoid has the power to turn all plants against humanity. Eventually they find the Hazel-Krynoid and Lucie is shocked to discover it’s as big as two houses! It’s now reached to the full height of the Biodome and can‘t extend any further without shattering the glass. Lucie says she’s run out of weed killer, but the Doctor knows it wouldn’t have made any difference anyway as the creature is getting ready to spawn. The voice of the Krynoid echoes around the dome and the Doctor responds, pleading with it to stop what it’s doing. The creature insists that there’s nothing left of Hazel, but Lucie remembers it said that once before and Hazel still reasserted herself for a while to stop the Krynoids from attacking. It happened just as the alarm went off and the Doctor realises it was the high intensity and frequency that weakened the Krynoid’s grip, allowing Hazel‘s mind to regain control. Lucie says she saw a fire alarm just by the door, but as she turns to leave, something snags her foot. The jungle is starting to fight back and the Doctor is soon caught too. He uses the last of his herbicide to release Lucie and tells her to leave him behind and set off the fire alarm. She manages to activate it and as the klaxon echoes around the dome, the Krynoid’s mental grip immediately begins to loosen.

The Doctor calls out to Hazel and tells her to fight against the controlling influence of the Krynoid. Lucie once again reminds her of what Alex Marlow did to her and urges her to concentrate on how much she wants revenge. Hazel has a moment of lucidity and the Doctor asks her to call together all the Krynoids and bring them to this Biodome. She agrees, on condition that the Doctor kills her as she doesn’t want to live like this. He promises to end all this, and while Hazel is summoning the other creatures, he shows Lucie the controls that regulate the atmosphere, temperature, pressure and humidity. He reconfigures the settings to increase the amount of carbon dioxide, methane and ozone and then switches the overhead lights up to maximum. He plans to deliberately recreate the greenhouse effect to dry out the air, increase the pressure and send the temperature soaring. It’ll turn the Biodome into a tinder box with the Krynoids themselves as the kindling.

The temperature is rising rapidly by the time the Krynoids finally arrive and the Doctor warns Lucie that under the increased pressure, the pipes in the dome are almost certainly going to explode violently. He tells her to get inside the airlock and promises he’ll follow in a moment. Marlow arrives and the Doctor realises he’s now fully possessed by the Krynoids. Marlow announces that he’s come to an arrangement with the Krynoids - they’ve promised to consume only the humans and leave the rest of the animal kingdom intact. This will be a world with no cars, no cities, no industry and no people. He thinks it’ll be paradise. The Doctor has remembered what it is he likes about the human race - they never give up and will always continue fighting against people like Marlow.

The Hazel-Krynoid turns on Marlow and tells him he’s going to die. Ignoring the man’s protests, the creature orders all the plants, branches and tendrils in the dome to tear his flesh apart. Lucie watches in horror as the entire rainforest unites to attack him. Hazel urges the Doctor and Lucie to go before she loses control and kills them too. They turn and race for the airlock - then dive for cover just as the entire complex explodes around them. When the dust settles, the Doctor confirms that the Krynoids have all been destroyed. Lucie says it was thanks to Hazel, which is just as well as she always wanted to save the world. The whole building is in flames and as there’s been no rain for months, the forest around them is like matchwood. The fire is bound to spread for miles, which should take care of any spores that might have escaped. Lucie is sad as the Krynoids were innocent people once and she suggests letting their families know what happened to them, but the Doctor thinks they’d be better off not knowing.