The fascinating 2002 film “28 Days Later” was crafted with equal parts restraint and audacity — zombie thrillers are rarely bathed in silence, much less on a scale as grand as a barren, lifeless London. In horror movie circles, Danny Boyle’s film was also lauded for giving zombies the ability to run after their prey rather than stalk them slowly. But while it was the sprinting undead that enticed many horror buffs to check out Mr. Boyle’s apocalypse-by-virus, the film’s real scares were found in the actions of the surviving humans, trapped and isolated in a world where the rules of society had vanished.

“28 Days Later” was a movie about awaking on doomsday and not knowing what to expect, first from an unknown global “rage” virus, then from the hundreds of thousands of infected, bloodthirsty souls roaming Britain, and ultimately from the few remaining survivors struggling to cope with the primal scenario of every-man-for-himself.

By contrast, in “28 Weeks Later,” the Juan Carlos Fresnadillo-directed sequel opening today, we see the original fear-of-the-unknown mutate into a fear of our own inadequacy in allowing history to repeat itself. The script, by Mr. Fresnadillo and Rowan Joffe, finds humanity shooting itself in the foot once again, arrogant enough to believe that we can win a second round over a force of nature we foolishly think we can control.

For those new to the franchise, an accelerated prologue (setting the tone for the frenzied visual style Mr. Fresnadillo will use during the many zombie attacks) brings viewers up to speed. Hiding out in a rural house, Don (Robert Carlyle) and a handful of survivors eat dinner and talk of the loved ones they have lost to the disease sweeping the hillsides. Don talks with his wife of their children, whom they managed to shuttle to a refugee camp. But it isn’t long before they are discovered in their hideaway and Don, who seems to represent all of humanity, is forced to make a horrific decision before escaping.

Twenty-eight weeks after the initial outbreak and devastation, the U.S. Army has landed in Britain, removed the corpses of the starved monsters, and declared that the “war against infection” has been won and that the reconstruction of the country can begin — first in a heavily fortified safe zone. It’s here that Don finds himself reunited, against all odds, with his children. In this armored corner of London, where the number of soldiers seems to equal the number of residents and where sharpshooters peer down at the population at all hours of the day and night, he tries to piece his family back together.

But this newly designed and meticulously protected military sanctuary is not the haven it appears. As it tends to do, human nature finds a way to buck best-laid plans, and as the virus finds its way back into this confined population, the army initiates “Code Red,” a horrific, ruthlessly designed protocol to isolate the outbreak.

Curiously, as we watch this virus spread among its first few hosts before going head-to-head with the most advanced human planning and technology, there’s something scientific — even anthropological — to this sequel that makes it a thinking man’s bloodbath. Indeed, even the basic nature of the disease, which can turn a loved one into a mortal enemy in a matter of seconds, leads to an interesting moral dilemma for these characters, who must learn to juggle empathy with savagery.

The first film had the appeal of working on a blank slate, of watching a journeyman as he learns a whole new set of rules. “28 Weeks Later” is more about watching nature lay waste to the most sophisticated human infrastructure designed specifically to repress it, and marveling at how many different variations of this theme Mr. Fresnadillo can conjure up — from depicting the breakdown of a confined populace into panic, the breakdown of a military chain of command, and the breakdown of the family unit, as Don and his children find themselves pitted against one another.

Following with the theme, Mr. Fresnadillo takes everything we’ve previously learned about the zombies — that they run, spit blood, and quickly multiply — and turns those rules against us, too. Gone are the scenes out in the rural neighborhoods, where zombies chase innocents from afar; here, the infected and the healthy are locked together in underground bunkers, and our jaws drop not because of the long-distance chase, but because we see, in gory, up-tight detail, what happens when these ravenous monsters have their dinner laid out in front of them.

It’s the cynicism that’s mesmerizing here. Forget awaking from a coma to discover a zombie-riddled world you don’t understand; “28 Weeks Later” jars us because it’s less about the monsters than about the way we can know the danger, prepare for the danger, and still fall short at the moment of truth. Damned if we do and damned if we don’t, all our weaknesses lead us down the same roads to the same cliff, and the same fall.