In a covert biological weapons laboratory in the Philippines, scientists work on a serum called Death One, which reanimates the dead. When Dr. Alan Holder and his assistant Norma experiment on a deceased human test subject, the corpse reanimates and reacts violently, prompting Dr. Holder to resign from the project. As he prepares to surrender the serum to the military, a group of rival gunmen ambushes the facility, with the lone surviving criminal absconding with Death One.
During the ensuing chase, gunfire breaches the container with the serum. Dr. Holder initially shows no concern about the virus escaping in this manner, given its inability to survive more than 30 seconds in the presence of oxygen, until he learns that the criminal accidentally touched the serum and became infected. General Morton and his soldiers arrive at the Sweet River Resort, where the criminal had fled, to find that the criminal had infected a bellhop, killed a maid, and cut off his own hand in a failed attempt to stop the infection's spread before finally succumbing to the infection. Morton orders the patrons and staff to be killed and buried in a mass grave, and the criminal's remains are transported to the facility's incinerator. As Dr. Holder and Norma fear, the ashes disperse into the air, infecting an entire flock of nearby birds.
Meanwhile, a trio of G.I.s on vacation are looking for girls and encounter a nearby bus, whose passengers include Joe, Nancy, Carol, Lia, Suzanna, Jane, Jane's boyfriend Tom, and bus driver David. Nearby, a tourist named Patricia and her boyfriend, Glenn, discover the dead birds, which reanimate and attack them. The birds assault the bus while the G.I.s attempt to evacuate it, and Lia is bitten. Patricia and Glenn stop at a nearby garage but flee when a machete-wielding zombie attacks Patricia, and the garage explodes after she sets the zombie afire. The G.I.s and the bus party stop at the now deserted Sweet River Resort. The men find a crate of guns left behind by the soldiers and begin fortifying the resort while Carol and one of the G.I.s, Bo, drive off to seek help.
When their car breaks down, Carol goes looking for water but is mauled by zombies and becomes zombified after her legs are torn off. Bo flees as more zombies emerge, eventually joining Patricia and Glenn. Glenn dies and reanimates as a zombie on the way to the hospital, killing Bo while Patricia escapes.
Back at the lab, Dr. Holder and his team start work on an antidote to Death One. Meanwhile, Morton's men initiate an operation to eradicate the zombies, now identified by the codename "locusts", referencing the zombies' behavior pattern of swarming from town to town.