In a dystopian future, humanity has suffered greatly from a disease brought on by a parasitic fungus that spreads through bodily fluids. Over the last decade, this infection has transformed the majority of its victims into feral, mindless creatures known as "hungries." Nevertheless, a glimmer of hope remains in the form of a cure lurking within a unique group of children who, despite being carriers, appear outwardly normal.
These children are kept in captivity on an army base, under the watchful eye of soldiers led by Sergeant Eddie Parks and subjected to experiments orchestrated by Dr. Caroline Caldwell. The children's education falls to Helen Justineau, who forms a particularly strong connection with a standout girl named Melanie. Their bond grows deeper, intensifying the stakes when Melanie becomes a target for dissection.
As chaos erupts and the facility is swarmed by hungries, Melanie escapes from her impending fate. She finds herself outside amid a brutal attack on soldiers. In a gripping turn of events, she and Justineau leap into an escaping truck alongside the injured Caldwell and two surviving soldiers, Gallagher and Dillon. To prevent any possibility of harm, Melanie is restrained and muzzled. However, tragedy strikes when a hungry ambush leads to the death of Dillon, and their journey becomes increasingly perilous when the truck suffers a breakdown.
On foot, the group makes the arduous trek to London, skillfully maneuvering through dormant hungries thanks to a "blocker" gel that disguises their scent. They seek refuge in an empty hospital for the night. During their stay, Caldwell discloses a shocking revelation: second-generation hungries, referred to as neonates, were birthed through gruesome means, indicating that these creatures still possess the ability to think and learn, forming a complex relationship with the fungus.
As dawn breaks, the group discovers they are encircled by hungries, but Melanie’s unique attributes allow her to pass unscathed, granting her the freedom to explore her surroundings. She scavenges for food, even consuming a stray cat, and proves vital to the survival of the group by luring away the hungries with a dog she finds.
Venturing further into London, they encounter ominous signs of the infection's impact, notably heaps of infected bodies around the BT Tower, which has succumbed to a monstrous fungal infestation. Caldwell warns that this growth can produce spores that threaten to annihilate humanity. They find shelter in a mobile lab previously dispatched by military forces.
Amid crises and dwindling supplies, Caldwell, gravely wounded by sepsis, decides she can secure the future of humankind by sacrificing Melanie in her research. During a desperate supply run, Gallagher is tragically killed by a band of feral hungry children, who cunningly lure unsuspecting victims. When Parks and Justineau find themselves trapped by the children, Melanie bravely confronts and defeats their leader, paving the way for the group's escape.
However, upon their return to the lab, Caldwell's intentions turn dark as she attempts to dissect Melanie. Faced with this reality, Melanie understands she is no mere experiment; instead, she embodies the evolution of humanity. In a pivotal moment, she escapes and ignites the towering fungal structure, unleashing a vast cloud of spores. Caldwell's pursuit ends in her own demise at the hands of the feral children.
As Parks searches for Melanie, he becomes infected by the released spores. In a heart-wrenching scene, he passes his weapon to Melanie, imploring her to shoot him before he changes into a hungry. With sorrow, she complies as he teeters on the brink of transformation. Inside the sealed lab, Justineau watches helplessly as the spores fill the air.
The film draws to a poignant conclusion with Justineau confined within the sealed lab, mourning her circumstances yet safe from danger. Outside, Melanie reigns over the hybrid children of the army base alongside the feral children, uniting them in a fragile peace. Justineau uses a microphone to impart knowledge to this new dominant species, embodying hope for a future amidst despair.
