In Antarctica, scientists Charles Winlett and Derek Moberley discover a pod buried in the permafrost, and take it back to their camp. John Stevenson, the base botanist identifies it as vegetable-based and estimates it has been buried in the ice for some twenty thousand years.
Back in London, Richard Dunbar of the World Ecology Bureau shows 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 believes it to be 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. The Doctor and Sarah arrive at the base. 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. 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. 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.
Moberly is killed by the now muated Winlett. Transformed into a Krynoid it flees the base and shelters in the outside generator hut. Scorby and Keeler -employees of Chase- hold the Doctor and others at gunpoint as they steal the remaining pod. Scorby and Keeler set up a bomb in the generator and escape in their plane. The Doctor and the others get free, but are attacked by the Krynoid, which kills Stevenson. The Doctor and Sarah flee the base as the bomb creates a chain reaction and destroys the whole area.
Regaining consciousness in the snow, the Doctor and Sarah are picked up by a team from South Bend in their Snow Cat vehicle. Meanwhile, Scorby and Keller return to Chase in England with the second pod. Dunbar is angered at how far Chase had gone to secure the pod, but warns Chase that the Doctor and Sarah are still alive, and are scheduled to meet with him and Sir Colin in two hours.
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. 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. The Doctor rescues Sarah and in the confusion, a frond from the pod stings Keeler's arm. 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.
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. She escapes back to the house and while in hiding, attracts Ducat's attention and asks her to take a message to Sir Colin. 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. They manage to escape to a cottage and barricade themselves in. 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 gets through to Major Beresford for assistance and sends Amelia home.
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 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. UNIT soldiers arrive and open fire with their laser gun, distracting the Krynoid so that the Doctor and his group can get to another door. The Doctor believes that Chase is possessed by the Krynoid and determines to find him and eliminate the threat he poses to them from within.
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. 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 and knocks her unconscious.
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.