A water-borne virus triggers a sweeping zombie outbreak across San Francisco, forcing a small group of survivors to hole up on Alcatraz Island. Dr. Lynn Snyder, Mariel Hemingway, joins the makeshift refuge, alongside others who range from scientists to ordinary caretakers, all clinging to a fragile sense of safety as they exchange rumors of a possible breakthrough. Communications from Dr. Arnold, French Stewart, a mainland scientist, hint at a potential cure, while Dr. Dan Halpern, LeVar Burton, struggles to advance his own work, studying whatever pieces of the infected he can access from the dead, a process hampered by the island’s limited resources. Caspian, Danny Trejo, and the other refugees burn zombie corpses stored on the island, trying to keep the outbreak contained in a place meant for safety.
A tide-laden horde crashes onto the island, sparking panic and forcing quick, desperate actions. The attackers are repelled, but not without losses: several refugees die, and two are infected, including Halpern’s daughter Julie, Kerisse Hutchinson. With hope wavering, the survivors decide to push for a route to the mainland, while Halpern stays behind to monitor the infected and continue his grim study. The situation deteriorates when Halpern is compelled to kill himself by detonating a grenade as his daughter bites him, a desperate attempt to spare him from the virus—and from a fate he can’t control.
Back on land, the group splits into factions because of diverging priorities. Snyder pursues the lead on Dr. Arnold, convinced that his research has yielded a cure, while Caspian’s faction focuses on scavenging supplies and pursuing what they believe to be an evacuation point. The split tightens as the infected numbers dwindle, only to reveal that the envisioned evacuation point has already been overwhelmed by the relentless undead. In a stark moment, Ashley, Heather Hemmens, chooses suicide by rolling a trolley into traffic, underscoring the mounting loss and the uncertain road ahead.
The remaining survivors—Snyder, Kyle Chad Lindberg, and Marshall Ethan Suplee—press forward and eventually locate Dr. Arnold’s refuge at the city’s water treatment plant, the place where the outbreak reportedly began. Arnold reveals that his tests on animal subjects have produced a vaccine, a glimmer of real hope amid chaos. With time running out, the trio makes a perilous dash to a rooftop helicopter, a moment of relief as they escape the city’s grip. Kyle and Marshall are briefly attacked, but Arnold’s vaccine proves effective enough to save them. As the group looks toward a new horizon, Arnold asks for a dedicated laboratory to manufacture the cure, holding onto a cautious optimism that science can still prevail and that a path to recovery might finally be within reach.
