Across the metropolis, children are dying, starved of oxygen. Columns of mantled beds fill public buildings, wartime casualties gasping for air. But this War is against insects, and the disease these innocents have contracted must be staunched by genetics. Scientists modify a hybrid bug, appropriately named the Judas breed. Infertility spreads through the roach kingdom, the plague is stilled, the scientists feted. Anyone who has slept in a cheap New York hotel will have reason to feel revulsion for cockroaches. They seem strangely unclean, their glistening brown bodies scuttling into shadows.

Three years pass. The two key scientists, played by Mira Sorvino and Jeremy Northam, marry, and evolution pulls off a betraying trick of its own, as someone or something commits a murder witnessed only by an autistic child. Northam is summoned to the death scene because there’s a bizarre entomological element to the crime, while Sorvino is approached by kids who have rescued a strange insect from the subway system. Events lure them into New York’s subterranean tunnels. Could the Judas bugs have found a way to adapt? Worse, if they have survived and grown to the point where they can lure their enemies with mimicry, could their enemy now be mankind?

Del Toro has several jaw-dropping instances of disbelief to overcome - big bugs, formula heroics, a motley group of survivors, a climactic effects-riddled showdown - and only the last (imposed by the studio) defeats him. For the rest, we get a truly skin-crawling experience for our money, mainly because this is the first director in ages to understand the dynamics of the horror film. He manages to make even the more lunatic elements of the plot ring true. The victims are the poor, the disenfranchised, the unnoticed. The enemy is remorseless, impossible to reason with, and genuinely frightening.

As in Del Toro’s previous film, Cronos, the focus is far removed from the traditional teen love interest, centring instead on an elderly man and a small boy marginalised by a city that couldn’t care less whether they live or die. Religious iconography abounds; there’s a suggestion that this new infestation has been brought upon us in biblical fashion, the price for our own betrayal of nature. All horror, at some level, requires a battle between good and evil, and here godlessness is matched with physical disgust. Shit hangs from the ceiling of the murder scene, and the creatures’ fluids must be smeared on open wounds to prevent the cast from being dismembered. The bravura set-pieces raise your feet from the floor: Sorvino opening the boys’ insect box in her darkened lab one rainy night, an unsettling alleyway encounter with what appears to be a man in a trench coat emitting guttural clicks, a terrifying swoop-attack along a deserted railway platform, and the unrepentant murder of a child, this last moment raising the hackles of American journalists who criticised Del Toro for succeeding too well in his task.

For four-fifths of its length, at least, Mimic is that great rarity, a genuinely eerie film; the incessant rainfall, the strange boy who mimics the creature lurking outside his window even as it copies him, the derelicts who survive beneath the city, a hidden population becoming less human than insects. Marco Beltrami’s excellent score adds a discomforting, jangly edge to the proceedings, and the CGI effects are used sparingly enough to convince. Visual surprises recur. There’s a sense, too, that the film mimics reality more closely than other recent horror outings because there are no cosy family units, no safe norms. Even Sorvino’s best friend, usually an anodyne role, has the odd habit of Polaroiding herself in moments of high emotion.

This is how people behave, at odds with the world, and how horror can advance, with ideas and intelligence. Catch it on the big screen if you can, on video if not.