Restless and isolated in a bleak room surrounded by bricks, a woman named Sarah anxiously stares at a calendar hanging on the wall. October is the month, and each day has been marked off. As she fixates on an image of a pumpkin patch, a terrifying sight unfolds: decaying arms burst through the wall, reaching for her.
Suddenly, she awakens from this nightmarish reverie, finding herself in the back of a helicopter, only to be confronted with the harsh reality of a crumbling civilization. Below her lies the desolate landscape of Fort Myers, Florida, where her team has landed near an abandoned harbor. Sarah is part of a small four-person squad on a mission to find any survivors of a catastrophic crisis that erupted almost a year ago, a mysterious plague that led to humanity's downfall. Her sometimes lover, Private Miguel Salazar, an overwhelmed soldier, shouts through a bullhorn for any remaining survivors as they stand amid decaying remnants of once-thriving communities. A newspaper blows in the wind, its headline starkly reading: THE DEAD WALK!, a somber reminder of the horrors that have unfolded.
In this grim new world, the dead rise to feast on the living, turning those bitten into zombies themselves, thereby swelling their ranks. Despite the efforts of the military and scientists, containment and cure have proved futile. Only the alligators, tarantulas, and land crabs survive in a city overtaken by the living dead, their cries echoing in response to the team’s distress calls, forcing them to retreat back to their sanctuary.
Their helicopter barely manages to land at a military compound encircled by chain-link fences, where they are met with a growing horde of the undead clamoring for entry. Among the soldiers assisting in securing the chopper is Johnson, immersed in the last pages of adult magazines, while Torrez cultivates marijuana plants. John, a composed Jamaican pilot, and his Irish radio operator friend, Bill McDermott, seem anxious to operate the nearby cargo elevator while urging the soldiers to refuel the helicopter.
Noticing Miguel's escalating stress amidst the chaos — he hasn't even disembarked from the helicopter — Sarah grows increasingly concerned. The group descends into the underground sanctuary, the Seminole Storage Facility, where they’ll be deprived of sunlight for days. Another military man, Miller, awaits their return, quickly extinguishing any remaining hopes with his remark that it was “another waste of time.” They are confined to cold, sterile white cubicles while John and Bill choose to stay in a nearby Winnebago, overlooking the eerie caves infested with both bats and captured specimen zombies.
Among their ranks, Sarah's team encounters new soldiers, Privates Steel and Rickles, who now oversee the task of capturing zombies for research purposes requested by the enigmatic Dr. Logan. Despite their exhaustion after a grueling 24-hour search, Sarah and Miguel agree to assist them on their latest mission into the caves. Tensions flare as Steel displays crude sexism toward Sarah, leading Miguel to take an unnecessary risk, almost costing them all their lives during a chaotic encounter with a female zombie. Eventually, Sarah manages to restrain the creature just in time to save Rickles.
In the dining area, Dr. Ted Fisher calls for better equipment for their experiments, while the unyielding Captain Rhodes challenges the research methods following the recent tragedies within their unit. Sarah confronts Rhodes about Miguel's mental state, but he refuses to take him off active duty, igniting disputes between them. Throughout the day, Dr. Matthew Logan, humorlessly nicknamed "Frankenstein" by the troops, delves into his grotesque experiments on the captured corpses, unveiling a shocking truth about the demise of Major Cooper, whose body Logan has appropriated for his research.
Frustrations boil over during a mealtime meeting led by Rhodes, who issues dire threats as supplies run scarce. As factions within the group clash, Sarah finds fleeting moments of solace in the camaraderie of John and Bill, contemplating escape from the madness engulfing them. Yet hope wanes as Logan's experiments are met with suspicion and disdain, culminating in a disastrous encounter that leaves Miguel wounded and traumatized.
A desperate attempt to capture more specimens ends in further chaos. Miguel's anguish ultimately drives him mad, leading to a catastrophic opening of the gates, unleashing a terrifying horde of zombies into their once-secure compound. In the ensuing pandemonium, the flimsy hold the remaining soldiers have on survival crumbles as they face the harsh, brutal reality of their undead-filled world.
As carnage reigns, Sarah, John, and Bill make a final stand against the relentless wave of the undead, confronting their own mortality and choosing to fight for any semblance of hope they can still cling to. They manage to escape only to be faced with the stark reality of dozens of zombies converging on their location and the uncertain fate of their helicopter.
But just as they confront the gathering threat, Sarah awakens once more, finding herself on an idyllic island with John and Bill fishing under the warm sun. On her calendar, she marks November 4th — the Day of the Dead — suggesting that the true horror may have only just begun or, perhaps, it was all just a nightmarish dream.
