Former UN employee Gerry Lane, along with his wife Karin and their two daughters, finds themselves trapped in the midst of a zombie apocalypse while stuck in Philadelphia traffic. As the situation rapidly deteriorates, the Lanes manage to escape to Newark, New Jersey, where they take refuge in an apartment belonging to a family with a young boy named Tommy. An old friend of Gerry's, UN Deputy Secretary-General Thierry Umutoni, arranges for their extraction via helicopter to a U.S. Navy ship anchored in the Atlantic, which is working tirelessly to understand the global outbreak.
Onboard, scientists, including Dr. Andrew Fassbach, theorize that the epidemic is caused by a virus and emphasize that developing a vaccine hinges upon tracing its origins. Through communication, they uncover that the term "zombie" was first used in an email from a military facility in South Korea that holds potential clues. Under pressure, Gerry reluctantly agrees to assist Dr. Fassbach in tracking down the outbreak after realizing that he and his family will be abandoned unless he helps.
Upon arriving at Camp Humphreys in South Korea, Gerry and Fassbach are met with an attack from the zombies. In a tragic accident, Fassbach slips and unintentionally shoots himself, leaving Gerry at the mercy of the base's remaining personnel led by Captain Speke. They discover that the infection originated from the base's doctor, who had contracted the virus while treating the initial patient. A CIA agent confined to the base advises Gerry to seek out safe grounds in Jerusalem, where the Israeli Mossad has established a secure zone prior to the outbreak becoming public knowledge.
As Gerry and his team rush back to their aircraft, another zombie attack ensues, resulting in casualties and Captain Speke's tragic suicide to prevent his transformation. Once in Jerusalem, Gerry encounters Jurgen Warmbrunn, the head of Mossad, who recounts how they had intercepted an Indian military communication about an ongoing fight against "dead spirits," warranting a quarantine around the city. However, the safety of Jerusalem is soon compromised when jubilant celebrations by survivors attract zombies to scale the walls, leading to chaos.
During the mayhem, Gerry witnesses zombies ignoring a starving boy, which piques his interest. He assists an injured soldier, Segen, by amputating her bitten hand just in time. The pair escape on a commercial flight, only to find themselves diverted to a World Health Organization facility near Cardiff, Wales after a zombie breaches their aircraft. In the ensuing chaos, Gerry manages to eliminate the threat but sustains injuries from the crash landing.
After regaining consciousness three days later, Gerry shares his theory with the remaining WHO staff: the infected seem to overlook those who are severely injured or terminally ill, as they are not suited for the virus to thrive. The team exercises this theory by purposely introducing pathogens into a controlled environment, leading Gerry to the pathogen vault. After an intense encounter with a zombie, he injects himself with an unfamiliar bacteria, successfully proving his hypothesis when the infected ignore him during his escape.
Ultimately, Gerry's family is reunited in a secure zone in Freeport, Nova Scotia, where a "vaccine" derived from harmful pathogens is developed, effectively acting as a disguise against the infected. With renewed hope, military operations against the zombies commence, and Gerry poignantly declares, > "This isn't the end. Not even close. Our war has just begun."
