Late one night, a group of British animal rights activists make a daring move to infiltrate a laboratory, aiming to liberate chimpanzees that are being subjected to medical experimentation. Despite warnings from a lab worker, they proceed with their plan and liberate the chimpanzees infected with the deadly Rage virus. This reckless act triggers uncontrollable chaos as the infected chimpanzees attack and infect the activists and a scientist present, leading to a rapid spread of the virus that transforms its victims into monstrous creatures in just a few moments.
Fast forward twenty-eight days later, we meet Jim, a bicycle courier played by Cillian Murphy, who wakes up from a coma in a deserted hospital after treatment for workplace injuries. Upon leaving the hospital, he finds himself in a hauntingly empty London, marked by signs of a catastrophic event. As he wanders through the eerie silence, desperately calling out for survivors, he stumbles across a church filled with what he believes are people, only to realize that they are, in fact, the very infected monsters he has been warned about. In a desperate run for his life, he is rescued by two other survivors: Selena, portrayed by Naomie Harris, and Mark, who lead him through the chaos to a safe haven in the London Underground, strategically employing explosive charges to evade the infected.
Selena and Mark explain the grim reality of Jim’s situation: while he was in a coma, the virus wreaked havoc, decimating the population and creating a lawless world dominated by the Infected. With his mind racing and his world shattered, Jim expresses a strong desire to find his parents. Although they believe this quest is foolhardy, Selena and Mark reluctantly agree to accompany him. Tragically, Jim discovers that his parents took their own lives during the onset of the infection. Later that night, as the group faces an assault from a swarm of Infected, Mark suffers a serious injury, leaving everyone in a state of panic. Despite his pleas for help, Selena takes immediate action, taking Mark's life to prevent the spread of the infection, emphasizing the nature of the deadly virus and its swift consequences.
As Selena and Jim navigate the desolate cityscape, the tension between survival instincts and human connection becomes a central theme. Selena insists that relationships are dangerous, prioritizing the struggle to stay alive amid the madness. Their journey leads them to a high-rise filled with Christmas lights, a glimmer of hope in their dark reality. However, as they venture up the building, Jim succumbs to a bad headache due to sugar withdrawal, and they narrowly escape another encounter with the infected thanks to a man in riot gear who helps fend them off.
Inside, they meet Frank, expertly portrayed by Brendan Gleeson, and his daughter Hannah. Welcomed into their home for the night, Jim and Selena grapple with the dilemma of staying with this newly formed group or continuing their solitary fight for survival. Despite Jim's belief that Frank and Hannah are good people, Selena warns him against misplaced trust that could lead to their demise.
As days pass, Frank reveals that essential supplies are dwindling, and they must leave the city for survival. He shares a recorded message from soldiers claiming to have a solution for the infection, but as they embark on a harrowing journey through a blockade, supportive camaraderie blossoms among them. They face numerous challenges, including a frantic tire change in a compromised situation with the Infected nearing their location.
However, their ordeal escalates when Frank inadvertently becomes infected, leading to a heartbreaking confrontation where he sacrifices himself to protect Hannah, an event that hurls Jim, Selena, and Hannah into the hands of a sinister military operation led by Major Henry West. The trio quickly realizes the horrifying truth behind West's so-called "solution" to the infection, which involves brutal control over female survivors.
As Jim plots their escape, he faces capture alongside Sergeant Farrell, who theorizes that the Infected may not be spreading worldwide as believed, narrowing their threat to the British Isles. Events take a darker turn as Jim, in a desperate bid for freedom, unleashes chaos within the military compound, leading to an intense sequence of action where the two worlds collide. Amidst bloodshed and engulfed in madness, he and Selena share a tumultuous but passionate moment, reaffirming their connection amid despair.
After a brutal confrontation with West, the survivors ultimately escape and manage to bring Jim to safety. Time passes, and Jim regains consciousness in a tranquil cottage, where they begin to find a semblance of peace. A hopeful moment arises as they create a large banner spelling "HELLO" in the meadow, signaling to the world above. With a passing jet offering a glimmer of hope as they wave, Selena muses on whether they were seen, encapsulating the struggle for hope and humanity in a world filled with darkness.
