A mysterious meteor shower blinds everyone who sees it, and also brings to Earth the spores of mobile and hostile Plant Aliens called triffids. Bill Masen, who is in hospital with his eyes bandaged due to an eye injury, ironically thus becomes one of the few people in Britain with intact eyesight; the film follows his struggles to find safety in the ensuing chaos.
In a subplot invented for the film, a marine biologist and his wife stationed on an island lighthouse struggle to find a way to defeat the triffids. In the end, almost by chance, they discover that the triffids are dissolved by sea water, and the world is saved.
This film contains examples of:
- Abandoned Hospital Awakening: Opens with the protagonist waking up in hospital following eye surgery, his eyes still bandaged, to find that nobody else is around.
- Adaptational Alternate Ending: The book ends with Masen and his friends holing up on an island off the British coast and resolving to keep fighting to take the world back from the triffids. The movie ends with Goodwin discovering a weakness by which the triffids can be easily defeated. It essentially gives the plot a "God will provide" ending similar to George Pal's The War of the Worlds.
- Adaptation Expansion:
- Brutally shows the horror of those first few days in two scenes; one shows an ocean liner, helplessly adrift, and the second an aeroplane pilot begging for someone to talk him down, as the entire crew is blind. The plane crashes near Masen's ship, but we never find out what happened to the liner.
- Adds a subplot of American marine biologist Tom Goodwin and his wife Karen (played by Janette Scott), who are trapped in a lighthouse because the relief ship didn't came as a result of the blindness and are eventually besieged by triffids.
- America Saves the Day: The American biologist Goodwin eventually fights off the triffids with the lighthouse's firehose out of sheer Last Stand desperation and thus discovers their the Weaksauce Weakness—sea water.
- Chekhov's Gun: The "SEA WATER — EXTREMELY CORROSIVE — FOR FIRES ONLY" water hose that is used by Goodwin to kill the triffids in the final act is shown very deliberately in an early scene.
- Death of a Child: Kept mostly to off-screen insinuation, but the scene with the airliner that crashes because of the pilot being blind and the passengers rioting when they figure it out prominently has a kid amongst said passengers (the air hostess tries to reassure the kid but the passenger right next to him instantly catches on that it means the pilot is blind, starting the riot, and the kid is tossed aside as they all scramble to the front of the plane).
- Easily Thwarted Alien Invasion: The triffids have the advantage for most of the movie, but it's over for them quickly once Goodwin discovers their weakness to sea water.
- Evil-Detecting Dog: One of these starts barking at a triffid, only to be killed by it offscreen.
- Heartwarming Orphan: Susan retains her sight because she was hiding in the baggage car all night, stowing away to escape the boarding school for orphans she was sent to. After Bill saves her from a panicked blind man, he will do anything to protect her for the rest of the film.
- Hollywood Acid: Although it's only used to dissolve the triffids and win the day, the big warning sign next to a hose saying "Sea water — extremely corrosive" suggests the writers believed sea water acts this way on everything.
- Kill It with Water: The triffids' big weakness turns out to be sea water.
- Moral Dissonance: Masen returns to the chateau to find sighted convicts holding the blind women at gunpoint and sexually assaulting them. He gets Christine Durant and Susan into the truck and drives away, making no attempt to save the helpless women. Even Durant — who earlier had vowed not to abandon the others — never mentions the chateau incident again.
- Plant Aliens: The original novel didn't establish where they came from (casually speculating on aliens and Soviet Super Science); the movie version explicitly made them into aliens.
- Promoted to Love Interest: The film leaves out Josella Playton, the character who was Masen's love interest in the novel, and replaces her with Christine Durant.
- Salt Solution: Salt water turns out to be the most effective weapon against the triffids.
- Saved by the Punishment: Most people were blinded by a meteor shower, but a group of prison convicts kept their eyesight because they were confined inside the prison.
- Screaming Woman: Janette Scott as Karen Goodwin. And despite that famous line from "Science Fiction / Double Feature" in The Rocky Horror Picture Show, she doesn't actually do any fighting.
- Slept Through the Apocalypse: Masen, in hospital with his eyes bandaged, sleeps through the meteor shower and its immediate aftermath, and wakes to find the world greatly changed.
- Survivor Guilt: Discussed by Bill and Christine after the chateau is overrun. Bill explains what shes feeling as, Why me? Why did I live and they didnt?
- Touch of the Monster: The advertising poster shows a triffid carrying a blonde woman in a brief white dress.
- Weaksauce Weakness: The movie ends with the discovery that the triffids can be easily destroyed with sea water.