The protagonist is Bill Masen, a biologist who has made his living working with triffids—tall, venomous, carnivorous plants capable of locomotion. Due to his background, Bill suspects they were bioengineered in the Soviet Union and accidentally released into the wild. Because of the excellent industrial quality of an oil produced by and obtained from the triffids, there is heavy triffid cultivation around the world.
The narrative begins with Bill in hospital, his eyes bandaged after having been splashed with triffid poison from a stinger. During his recovery he is told of an unexpected green meteor shower. The next morning, he learns that the light from the unusual display has rendered any who watched it blind (later in the book, Bill speculates that the "meteor shower" may have been orbiting satellite weapons, triggered accidentally). After unbandaging his eyes he finds the hospital in chaos, with staff and patients without sight. He wanders through a chaotic London full of blind inhabitants and meets wealthy novelist Josella Playton, whom he rescues after discovering her being forcibly used as a guide by a blind man. Intrigued by a single light on top of the Senate House in an otherwise darkened city, Bill and Josella discover a group of sighted survivors led by a man named Beadley, who plans to establish a colony in the countryside. They decide to join the group.
The polygamy implicit in Beadley's scheme for rebuilding society appalls some group members, especially the religious Miss Durrant. However, before these plans can be put in place, a man named Wilfred Coker stages a fire at the university and kidnaps a number of sighted individuals, including Bill and Josella. They are each chained to a blind person and assigned to lead a squadron of the blind, collecting food and other supplies, all the while beset by escaped triffids and rival scavengers.
Soon Bill's followers begin to fall sick and die of an unknown disease. When he wakes one morning to find the survivors have left him, he returns to the Senate House to seek Josella but his only lead is an address left behind by Beadley's group. Joined by a repentant Coker, Bill drives to the address, a country estate called Tynsham in Wiltshire. He finds part of the Beadley group, now led by Durrant, who eventually tells him that Beadley went to Dorset a few days before he arrived; there has been no sign of Josella. Bill and Coker decide to follow Beadley, finding small groups of blind and sighted people along the way. Eventually they decide to separate, Coker returning to help at Tynsham, while Bill heads for the Sussex Downs after remembering a remark Josella made about friends she had there.
En route, Bill rescues a young sighted girl named Susan, whom he finds trapped alone at home, while her young brother lies dead in the garden, killed by a triffid. He buries the boy and takes Susan with him. A few days later, during a night of heavy rain, they see a faint light in the distance. Upon reaching it, they discover Josella and her friends.
The survivors attempt to establish a self-sufficient colony in Sussex with some success but are constantly under threat from the triffids, which mass around the fenced exterior. Several years pass, until one day a representative of Beadley's faction lands a helicopter in their yard and reports that his group has established a colony on the Isle of Wight. Durrant's talk of Dorset was a deliberate attempt to throw Bill off on his journey to find Beadley. While Bill and the others are reluctant to leave their own settlement, the group decide to see the summer out in Sussex before moving to the Isle of Wight.
Their plans are hurried by the arrival in an armoured car of the militaristic representatives of a self-appointed, despotic government. Bill recognises the leader as a ruthless young man he had encountered on a scavenging expedition in London, whom he had watched cold-bloodedly execute one of his own group who had fallen ill. The latter plans to give Bill a large number of blind people to care for and use on the farm as slave labour; he will also take Susan as hostage. Feigning agreement, Bill's group throw a party, during which they encourage the visitors to get drunk. Creeping out of the house whilst the visitors are fast asleep, they disable the armoured car by pouring honey into the fuel tank and drive through the gates, leaving them open for the triffids to pour in. The novel ends with Bill's group on the Isle of Wight, determined one day to destroy the triffids and reclaim their world.
