After a blinding global meteor shower (possibly another military accident...) the vast majority of the human race was then blinded, thus ensuring the collapse of our 20th century technological civilization and the total domination of the planet by man-eating, mobile triffids. Bill Mason, who's eyes had been bandaged during the meteor shower, was spared this macabre fate, as were a few others (including soldiers stationed on submarines...), and together the survivors had to reckon with the terrifying post-apocalyptic world.
Among other things, Wyndham's novel served as an explicit critique of the Cold War (particularly the shadowy veil of secrecy surrounding the Iron Curtain). On perhaps a deeper thematic and social level, the book also revolved around the growing pains of a new world order, and even touched on controversial subjects such as polygamy.
A film adaptation of The Day of the Triffids from scenarists Bernard Gordon and Phillip Yordan and director Steve Sekely played in cinemas in 1962. It's generally considered a classic to the over- thirty-year-old crowd because - heck - we grew up with it. In reruns on television, primarily. Franky, looking at the movie today, you can detect it is a low budget effort with extremely limited effects. Nonetheless, The Day of the Triffids boasts a tremendous sense of scope (thanks, in part, to the use of several highly creative and nearly invisible matte paintings.) Still, it's difficult to deny that this 1960s take on the material proved a bit less provocative than Wyndham's source material.
Specifically, the triffids are here tagged as being extraterrestrial in origin, rather than the result of CCCP genetic tinkering. There's actually no reference to the Soviet Union in the film whatsoever, and many of the characters have been been dramatically altered, though Bill Mason -- here an American naval officer -- remains our protagonist. The Cold War commentary is totally missing, and that's a disappointment. Furthermore, much of Wyndham's sociological material (namely polygamy, and the absolute necessity of polygamy to repopulate the species) is also excised.
In place of these elements, the film version of The Day of the Triffids adds a subplot revolving around married scientists, Tom and Karen Goodwin (played by Kieron Moore and Janette Scott), battling Triffids on an isolated island. The husband is an unhappy alcoholic, toiling away in a lighthouse with his concerned wife. When the triffids attack, hubby shrugs off the whiskey and recommits himself to science in an effort to destroy the Triffid infestation and save the planet. Quite by accident, Tom discovers that sea water (salt water) dissolves the beasts. This too is a significant alteration from the novel, which offered no simple solution to the Triffid dilemma.
Though much of Wyndham's original material has been jettisoned, it's not fair to state that Day of the Triffids is entirely devoid of resonant or meaningful themes. In particular, it depicts quite ably not only man's battle against Mother Nature (the evil Plants), but also against his own human nature. There's one riveting sequence, for instance, set in rural France, in which escaped convicts (still possessed of sight) attack a girl's school and attempt to force themselves on the blind girls living there. As the trailer puts it, "civilization disintegrates into primitive animalism!" Yikes!
Yet I also admire The Day of the Triffids because it balances the darker view of humanity at his worst (exploitative, alcoholic, and defeatist) with one showing him at his best. There is hope and commitment in this world, dramatized particularly in regards to the birth of a baby at a Spanish villa, and Mason's decision to help bring this child into the world safely.
Also, the isolated lighthouse scenes, though reputedly added at the last minute to grant the film an adequate running time, succeed in raising the tension quotient considerably. In movie terms, this is a classic "siege" scenario (see: Night of the Living Dead): angry, mobile Triffids threaten to break into the lighthouse at every turn, and Tom and Karen have nowhere to run, nowhere to hide. At one point, they are overrun and left to flee up a staircase to the top of the lighthouse. The triffids pursue, slowly but surely. These claustrophobic moments help the film live up to the trailer's description of the film as a "flesh-crawling experience in terror."
Critical objectivity requires that I acknowledge some pesky flaws here. The Day of the Triffids starts off slow (really, really slow...), with a relatively stupid narration that explains to the audience the obvious concept of carnivorous plants. ("There are certain plants that are carnivorous...are "eating" plants," intones a baritone-voiced narrator with utter seriousness).
Also, the opening Triffid attack on a night guard at the Royal Botanical Gardens is laborious drawn-out, and tends towards silliness. The guard appears to be mesmerized by the Triffid, and wanders into the waiting branches of the beast. It should have looked like it yanked him in, not like he wanted to give the Triffid a hug.
But after approximately thirty-minutes or so, The Day of the Triffids really picks up, and makes the absolute best of an extremely limited budget. In depicting a worldwide holocaust, the film efficiently (and economically) depicts what occurs when mass blindness afflicts a plane in flight, and a ship at sea. We see cities and military installations on fire (via models, mattes, and rear-projection work). There is a beautifully-orchestrated chase sequence (replete with that genre convention: the car that won't start) set in a misty swamp, as a mammoth Triffid uproots itself, and crawls up out of the bog in pursuit of Susan. Another moment, with a slimy Triffid methodically crawling up the lighthouse staircase step by step (as Tom and Karen sleep, unawares) is also suspenseful.
Yet what remains jaw-dropping about The Day of the Triffids is the manner in which the film successfully projects an epic sense of scope. There are awe-inspiring compositions aplenty. My favorite shot depicts thousands of hungry Triffids gathering at an electrified fence, while Mason tries to fight them back with a flame thrower. The high-angle imagery is just right, as the Triffids - en masse - move and caw and click dramatically, and Mason wages what appears to be a hopeless campaign against them.
The Day of the Triffids depicts a world-wide meteor storm, a train wreck, a plane crash, military bases aflame, vast metropolitan centers devoid of life (in scenes that seem to forecast images in films such as Day of the Dead  and 28 Days Later ) and also makes the threat of walking. man-eating plants palpable...and by the climax, totally believable. That's no small accomplishment, and the sense you get watching this film is that everybody - from director and actors to the special effects artists - truly committed to the project. They stretched their miniscule budget as far as it could possibly go, deploying ingenuity to fill the gaps.
I've said it before, but it bears repeating: it isn't entirely unreasonable, in my view to give a somewhat flawed film an A for effort. The Day of the Triffids -- even with some occasionally dim-witted moments -- really goes for the gusto. And it succeeds more often than it fails. Therefore I believe it earns the long-standing reputation as a classic of the genre. A caveat, of course: this film in no way could be considered more than modereately faithful to the Wyndham novel. If you're looking specifically to recreate that experience, you may be disappointed in the movie. But if you're looking for a good, post-apocalyptic horror film from the 1960s, one with an unusual and memorable antagonist as well as some resonant images of mankind's fall from grace, this movie fits the bill.