When two bumbling employees at a medical supply warehouse accidentally release a deadly gas into the air, the vapors cause the dead to rise again as zombies.
A group of kids discover one of the drums containing a rotting corpse and release the 2-4-5 Trioxin gas into the air, causing the dead to once again rise from the grave and seek out brains.
Curt Reynolds steals his father's security key card, and he and his girlfriend, Julie, explore the military base where his father works. They observe Curt's father, Colonel John Reynolds, Col. Peck and Lt. Col. Sinclair overseeing an experiment with a deceased body.
Set 10 years after the events of ''Return of the Living Dead 3'', the film starts out with Charles Garrison (Peter Coyote) traveling to Chernobyl to collect the last 6 canisters of Trioxin 5. Two Russian sellers take him to an empty power station which contains the Trioxin. One of the Russians get Trioxin on his hands, turning him into a zombie. Upon hearing screams, Garrison runs to see the zombie eating the other Russian partner. Garrison promptly shoots the zombie in the head.
The film opens a year after the previous film with Charles Garrison arriving at a mortuary with one of the canisters of Trioxin that he salvaged. He is greeted by a team of Government of Russia officials whose goal is to destroy the last of the canisters to avoid another incident. Nevertheless, one of them sprays three corpses with the gas, and revives them. Charles is killed during the incident, along with the mortuary owner and one of the government officials.