Back in London, Richard Dunbar of the World Ecology Bureau tries to show the Doctor photographs of the pod. Although he feels that the Doctor cannot help them, his superior Sir Colin Thackeray insisted. The Doctor examines the pictures and asks if they had considered that the pod is extraterrestrial. He tells Dunbar to contact the expedition by their regular video link, and tell them not to touch it until he arrives.
Back at the base, Stevenson discovers that the pod is growing larger and he believes it is absorbing ultraviolet radiation. In England, Dunbar visits the estate of millionaire Harrison Chase. Chase's estate is filled with thousands of plants, and he considers it his mission to protect the plant life of Mother Earth. Dunbar has come to show him pictures of the pod, its possible extraterrestrial origin and hints that such a valuable specimen could easily disappear.... for a price. Dunbar gives Chase the location of the pod. Chase calls for one of his men, Scorby, telling him to take Keeler along.
At the base, Winlett is half asleep near the pod when it opens up. A frond-like tentacle whips out and stings his arm, causing Winlett to collapse in pain. When Stevenson and Moberley find him, Winlett's face is covered with green hives. A message is sent to London informing them of the situation, but with the bad weather, it will take a few days for a medical team to reach the base.
Meanwhile, the Doctor and Sarah arrive at the base by helicopter and are met by Moberley. He tells them that usually up to a dozen people live at the base but currently the others are sixty miles away at South Bend, measuring the ice cap. In the sickbay, Winlett's body temperature and pulse are dropping rapidly. His face and body are now covered with a green fungus, and its growth is accelerating. Mike Wilson from South Bend radios them that the medical team has turned back due to whiteout conditions.
The Doctor asks for a blood test on Winlett, who is growing increasingly monstrous, and examines the now-empty pod. Stevenson acknowledges that it may be his fault; convinced that the pod was alive, he placed it under a lamp. Outside the base, the Doctor digs at the ice, uncovering another pod. Noting that the pods travel in pairs, the Doctors transfers it to the base freezer. On analysis, Winlett's blood is found to contain no blood platelets, but instead has schizophytes — microscopic organisms akin to plant bacteria.
The sound of engines is heard, and Moberley and Stevenson go meet what they presume is the medical team. The Doctor tells Sarah that Winlett is turning into a Krynoid, a kind of galactic weed that settles on planets and eats the animal life. He does not know exactly how the Krynoids travel between the stars; their planet may be geologically turbulent, with surface explosions sending matter shooting into space.
Stevenson and Moberley escort two men —Scorby and Keeler— into the base. The new arrivals claim that their private plane got lost and wound up at the base. The Doctor leaves to check on Winlett, taking the others and leaving Scorby and Keeler alone.
Winlett's transformation is nearly complete. The Doctor suggests that the arm be amputated to try to stop the spread of the infection at its source. Even if it does not work, it might buy them time. Moberley, as zoologist and therefore the most qualified to do the operation, is hesitant, but Sarah persuades him in the end. As they leave to gather the necessary equipment, Winlett rises from the bed.
When Moberley starts back towards the sickbay, he spots Winlett leaning against the wall. As Moberley approaches, Winlett reaches out and strangles him…
Sarah finds Moberley's body. Stevenson cannot believe that Winlett has done this, but the Doctor tells him that Winlett no longer exists; his mind has been taken over, and soon he will turn completely into a Krynoid. They gather their things to search for the creature and take the rifle, not realising it has been sabotaged. When the others leave, Scorby and Keeler go in search of the pod.
Dunbar tells Chase about a report of an infection at the base, but is shocked when Chase tells him he has sent his own men to Antarctica. Dunbar seeks an assurance that should anything go wrong, Chase will protect his identity. Chase pays Dunbar off.
The Krynoid finds the generator hut, built far from the base because of its new fuel cell system. However, when it sees the Doctor, Stevenson and Sarah approach, it turns away. Unable to find the Krynoid, and hoping that it is frozen in the snow, the three head back to base, and the Krynoid seeks shelter in the hut after their departure.
Meanwhile, Scorby and Keller ransack the base. When South Bend calls about the medical team, Scorby pretends to be Moberley and asks them to cancel it. When South Bend gets suspicious, Scorby disconnects the radio. Keller finds the open pod, and realises that it has germinated. Scorby wants to find the actual plant that emerged, but they find Moberley's body in the sickbay instead.
When Sarah and the Doctor enter the sickbay, Scorby holds them at gunpoint. Scorby questions the Doctor about Moberley, and the Doctor tells them about the pod and Winlett's infection. When Stevenson discovers them, he finds his rifle useless and is put with the others, but lets slip that a second pod exists.
Scorby threatens Sarah's life and the Doctor relents, telling Scorby that the second pod is in the freezer. Stevenson is forced to take it out, placing it in a thermal container for storage. Scorby then forces Sarah to take him to the generator hut with Keeler in tow, leaving the others tied up behind.
The Doctor knocks over an unlighted kerosene lamp and uses the broken glass to cut Stevenson's bonds, not knowing that Scorby has planted a bomb in the hut which will blow up the base. Keeler gets cold feet and tries to stop Scorby, who tosses him out of the hut.
Freed, the Doctor tells Stevenson to contact South Bend for the medical team, and goes in search of Sarah. However, the Krynoid has returned to the base, and kills Stevenson while he is making the call. The Doctor sees Scorby and Keller's plane take off, and runs to the generator hut where Sarah is tied up. She tells him about the bomb, but the creature arrives as well. They manage to get out the door and lock the Krynoid in, running away as the hut explodes, taking the rest of the base with it…
At the meeting, the Doctor and Sarah describe how well planned the theft of the pod was, but the Doctor believes that the two men were stooges. The discovery of the pod had only been reported to the Ecology Bureau, so the leak must have come from them. The Doctor is sure that the pod is in England, and says that if they do not find it, it will be the end of everything — even Sir Colin's pension. He tells Dunbar to arrange for him to go to the Botanic Institute.
As they leave the building, a driver meets them claiming to be their ride. However, the limousine stops in the countryside, and the driver orders them out at gunpoint. With a bit of teamwork, the Doctor manages to jump the driver and punch him out. The Doctor and Sarah search the car, and find a painting by Amelia Ducat, one of the world's leading flower artists. When they visit her, Ducat tells them that the owner of the painting is Harrison Chase, and that he never paid her for the painting.
Keeler, who is a botanist himself, unsuccessfully tries to convince Chase to stop the experiments on the pod. Chase orders him to inject the pod with fixed nitrogen. Dunbar calls Chase and tells him that his driver is in the hospital, so when the Doctor and Sarah try to sneak into the mansion, they are spotted by some guards and Scorby, who capture them.
The Doctor and Sarah are brought before Chase, and despite having Scorby's gun at his head, the Doctor asks Chase grimly to hand over the pod. Chase politely refuses: He has the greatest collection of plants in the world, and when the pod flowers, it will be his crowning achievement. Before he executes them, Chase decides to show the Doctor and Sarah around the mansion, and his plant laboratory.
Keeler notes that the pod is growing, and tells Hargreaves, the butler, to summon Chase to the annex. There, Chase tells Keeler to inject more nitrogen into the pod. Scorby escorts the Doctor and Sarah into the gardens to kill them, but the two manage to overpower Scorby. The Doctor uses rope to lower Sarah down the wall so she can go and warn Sir Colin while he returns to the house to examine the pod. However, Sarah gets captured again.
The Doctor makes his way back into the mansion while Sarah is escorted by Scorby back to Chase. The Doctor watches through the skylight, horrified as Chase orders Sarah forced down to a chair, grabbing her arm and pinning it next to the pod. He wants to know what happens when the Krynoid touches human flesh. As Sarah struggles, the pod begins to open…
Keeler begs Chase to get him to a hospital, but Chase is more fascinated with the transformative process than saving Keeler's life. Chase and Hargreaves take Keller to the nearby cottage. Chase observes that the growth seems to be absorbing protein from Keeler's body, and tells Hargreaves to keep Keeler fed with meat.
When the Doctor returns to the empty laboratory, he is captured by Scorby and a guard, who take him to the compost room. Scorby activates the crusher, remarking that Chase recycles everything. The main gate calls the house: Amelia Ducat is here demanding her money. To avoid a fuss, Chase agrees to see her. Meanwhile, Sarah has entered the cottage, and sees Keeler, who is still lucid, although covered with the Krynoid growth. Keeler tries to convince Sarah to free his bonds, and when Sarah refuses, he grows more angry and paranoid. Sarah hides in a cupboard when she hears Hargreaves approach with a meal, and leaves at the first opportunity.
In the Library, Chase writes Ducat a cheque. Scorby enters and asks if they should start the "recycling experiment", and Chase says he wants to see it, asking Scorby to escort Ducat out. Chase goes to the compost room, and reveals that he has been feeding Keeler. The Doctor, tied up, is placed in the machine, to be shredded and pumped into the garden.
Sarah hides behind a suit of armor as Ducat and Scorby pass. She attracts Ducat's attention and asks her to take a message to Sir Colin, going back into hiding when Scorby returns to show Ducat out. Outside, Ducat enters a car with Sir Colin and Dunbar inside, and tells them what Sarah said. Dunbar, realising he has made a terrible mistake, says he will go in and get the Doctor. He tells Sir Colin that, if he does not return in half an hour, to return to London and call UNIT.
Sarah finds her way into the compost room and turns off the crusher just in time to save the Doctor. Hargreaves finds that Keeler has now almost completed his transformation, and runs in a panic as the creature frees itself. In the mansion, Dunbar pleads with Chase to abandon the experiment as Hargreaves reports Keeler's transformation to Chase. Dunbar says that this has gone far enough, and he is going to get help. Chase tells Scorby to stop him.
Scorby pursues Dunbar through the grounds as the Doctor and Sarah find Keeler missing from the cottage. The Doctor takes a sword from over the fireplace and they leave to search for the Krynoid. Dunbar runs into the monster, which is far larger than the Winlett creature was, and no longer even humanoid. He shoots at it uselessly, and is held fast by the surrounding plant life as the Krynoid kills him. Dunbar's screams attract the attention of Scorby and the guards as well as the Doctor and Sarah. The latter get there first, the Doctor drawing the sword above Dunbar's body as the Krynoid lurches towards them…
The Krynoid smashes a window and extends a tentacle inside but they manage to drive it off. The Krynoid speaks using Keeler’s voice, demanding that the Doctor come out and join it and it will spare the others. Scorby is more than willing to give up the Doctor until Sarah Jane points out that without the Doctor they have no chance. The Doctor suggest Scorby rig up a bomb so they can all escape while the Krynoid is distracted.
Sir Colin tries to get through on the phone while Amelia gives him some advice. Sir Colin tries to contact the Brigadier and UNIT but Lethbridge-Stewart is out of the country. He eventually gets through to Major Beresford for assistance and sends Amelia home.
Chase prepares to take photographs of the Krynoid, claiming he has nothing to fear from it. The Doctor and Scorby initiate their plan as Chase arrives outside. Scorby throws his improvised bomb out an upstairs window and the Doctor makes a run for it. The Krynoid goes in pursuit, but the Doctor escapes in the limousine, leaving the Krynoid behind.
Scorby tries to find Chase at the greenhouse and discovers where he is from Hargreaves. They begin barricading the windows in preparation for the Krynoid’s attack. Chase makes his way through the grounds and confronts the Krynoid. It notices him and he approaches, taking photographs. It moves toward him as Chase claims he doesn’t mean it harm.
The Doctor arrives at the Bureau as Major Beresford warns he can’t do anything without evidence. The Doctor warns the Krynoid can channel its power through other plants, turning vegetation against humans. He shows them a series of reports of deaths of people near Chase’s estates being killed by plants. He then calls Sarah Jane and tells them Beresford is preparing to attack the Krynoid with a laser gun, but the Krynoid cuts the phone wires.
Chase wakes up in the woods and has clearly had a major experience: He begins talking about how the world will be recreated.
Scorby arrives and Sarah Jane points out how the vegetation around the windows is getting thicker. When Hargreaves reports that he heard screams, Sarah Jane taunts Scorby into going to investigate with her, and they find a dead guard killed by plants. Chase arrives and tells them that it’s the plants’ world, and humans are only parasites. He goes to the manor to develop his photographs, then begins speaking to the plants in his greenhouse.
Scorby, Sarah Jane, and Hargreaves go in to confront Chase and he speaks of how the world will be made perfect. They can’t get through to him as he talks about how he is one with the plants and animals are the enemy. Sarah Jane notices that the plants are closing in on them. The Doctor and a UNIT soldier drive onto the grounds while the plants overwhelm Sarah Jane and the others and start to strangle them.
The Doctor and the UNIT soldier, Sgt. Henderson, arrive and brought chemical plant-killer. They dispose of the plants, saving Scorby and Sarah Jane while the older Hargreaves is dead. Chase runs away and the Doctor and the others make their way into the lab and start removing the plants. But once they're outside Chase locks the door behind them and they can only gaze in horror as the now enormous Krynoid towers over them....
After they leave, Chase slips back into the laboratory and destroys the loudspeaker system. Unable to find the millionaire, the others return to the laboratory and the Krynoid tries to break its way in. Henderson goes to get more timber to barricade the windows and Chase knocks him out.
Scorby starts to panic and wants to run, but the Doctor warns him that every plant on the grounds is under the Krynoid’s control. Meanwhile, Chase puts Henderson in the compost machine and activates it, killing the unconscious soldier.
The Doctor works to repair the loudspeaker system as the Krynoid renews its attack, and Scorby panics and runs for it. He tries to make his way across the grounds through fields of hostile plants and makes his way across a shallow pond, but the plants grab and pull him underwater, killing him.
The Doctor and Sarah Jane realize that Henderson is gone and Sarah Jane goes to look for the soldier. She makes her way to the compost machine room and Chase confronts her, telling him he’s become part of the plant world thanks to the Krynoid. Chase plans to support the Krynoid and refers to humanity as parasites, then attacks Sarah Jane.
Beresford contacts the Doctor, who warns they have 15 minutes until the Krynoid germinates, spreading its seeds across England. The Doctor tells them to launch an air strike before it’s too late, and regardless of the fact he and Sarah Jane are at ground zero.
Chase has tied up the unconscious Sarah Jane and starts feeding her into the compost machine. The Doctor arrives, sends Chase flying, and shuts off the machine to untie Sarah Jane. He gets her out but Chase turns the machine back on and throws himself at the Doctor, and the two struggle inside the machine’s bin. The Doctor manages to climb out as Chase is pulled into the machine despite the Doctor’s efforts to save him.
The RAF launches a sighting run as Beresford and Sir Colin look for any signs of the Doctor. Sarah Jane and the Doctor can’t get out through the plant life covering the house but the Doctor rigs a steam pipe and they blast their way out. They make their way through the hostile plant life and take refuge in a clearing filled with cut-down trees, as the RAF opens fire and destroys the Krynoid along with the mansion.
Later, the Doctor, Sarah Jane, and Sir Colin discuss the apparent destruction of the Krynoid. Sir Colin wants the Doctor to address the Royal Horticultural Society but the Doctor instead offers to take Sarah Jane on a vacation to Cassiopeia. However, the TARDIS materializes in Antarctica when the Doctor forgets to reset the coordinates.
The Doctor's dialogue with Amelia Ducat about the car boot and model homages Oscar Wilde's The Importance of Being Earnest.