A Special Forces team is sent to an apartment complex to rescue a reporter named Ángela and destroy the building. After an attack by firefighters and infected residents, the survivors of the team, Lucas and Guzman, complete the mission. Ángela wakes up on a ship, chained to her bed, with no memory of anything. Dr. Ginard takes a sample of Ángela’s blood for testing. Lucas and Guzman also on board the ship, which has been effectively quarantined due to isolation. After a failed escape attempt, Ángela is informed by Doctor Ricarte that she is not infected. Meanwhile, Guzman becomes acquainted with Captain Ortega and technician Nic, who reenact the events in the apartment complex with the help of Ángela’s camcorder. Nic sends the film to the doctors in their laboratory, where they begin experimenting with the pathogen.

The doctors discover that their test subject, an infected monkey, was released during a power outage the night before. The monkey infects the ship’s cook, who in turn contaminates the food. Ingestion infects much of the crew and guards. After a failed attempt by the doctors to cure the cook with an antidote, Ángela, Guzman, Lucas, and an elderly woman who survived an outbreak at a wedding arrive on the bridge. Along the way, the old woman becomes separated from the group, and Lucas stays behind to save her, only to find her infected, forcing him to shoot her down. Ricarte realizes that his attempt to find an antidote has failed because it requires a non-mutated strain of the pathogen. Suddenly, before he can initiate a self-destruct sequence, his assistant informs him of the reconstructed film, which shows Ángela being infected with a worm-like parasite from the original carrier. Ricarte attempts to remove the parasite by forcing Ángela into surgery, who is saved when an attack by the now-infected Ginard allows her to escape. Ricarte searches for Ángela, who ambushes and bites him. Although his blood scans come back negative, Ricarte believes that Ángela is tricking him. It is revealed that during Ángela’s rescue, the parasite transferred to Guzman, who trapped Ángela in the bowels of the ship as Ricarte initiates the ship’s self-destruct sequence.

Lucas and Nic attempt but fail to start the ship’s auxiliary engine, and Lucas is killed by the infected. Nic finds Ricarte trying to escape on a lifeboat. Nic knocks Ricarte unconscious, takes the lifeboat for himself, and goes in search of Ángela. Nic finds her running away from the infected monkeys, who kill them with an outboard motor. They are then attacked by Guzman, who attempts to transfer the parasite back to Ángela. Just before they succeed, Nic and Ángela kill Guzman with a harpoon gun. Ángela and Nic leave the boat as all the infected chase them. Ángela jumps into the sea and Nic follows, escaping as the ship explodes. Underwater, the parasite slowly writhes and is swallowed by a fish. In the credits, Ángela and Nic are shown in a taxi, having arrived home.