At a run-down, understaffed hospital, Dr. Akiba, Koichi Sato, refuses to admit a patient with a strange black rash, and he is soon pulled into a severe crisis in Room 3 when a burn victim dies after being given the wrong drug. In the aftermath, Akiba, Dr. Uozumi, and four nurses decide to cover up the circumstances of the death and move the body to an unused room, hoping to keep the incident quiet.
The situation takes a troubling turn when the head nurse, Kaho Minami, discovers that the patient Akiba refused to admit has been left in the hallway and informs him. Yet when Akiba investigates, he finds that Dr. Akai, Shirô Sano, has taken the patient and intends to study his symptoms. The patient, though still alive, shows a disturbing change: his body mass is liquefying into a creeping green goo. Uncertain how much Akai knows about Room 3, Akiba and the others reluctantly agree to help examine the patient.
Their efforts are interrupted when they return to the room to find the patient missing and the head nurse unconscious. When she comes to, she begins acting strangely while bleeding green goo from her ears and eyes, signaling a spreading infection that may be beyond their control. The staff scramble to contain the risk, placing the head nurse on a bed and wrapping her in plastic to try and limit the contamination.
The tension escalates as the mean nurse encounters the young nurse, who is drawing her own blood. The conversation spirals into chaos when the younger nurse lets out a maniacal laugh and drives two needles into her own body, spraying the other nurse with the same ominous green goo. The situation darkens further when Uozumi, while speaking to a patient he had killed earlier, is overwhelmed by guilt; Akiba arrives to find Uozumi alone, his eyes turning white and green goo seeping from him.
Aghast, Akiba points to the infected mean nurse—now also covered in green goo and hanging upside down from the ceiling—before fleeing the room. He later confronts Akai, accusing him of creating and spreading the virus. Akai denies responsibility and explains that the infection spreads on a mental level, infecting the subconscious of those affected. He urges Akiba to remember what happened in Room 3 earlier that night.
Dr. Nakazono, Michiko Hada, enters and finds Akiba talking to himself, forcing him to confront that he has been speaking with his own reflection. She discovers the corpses of the two nurses—dead, their bodies bloodied and devoid of the visible green goo—and realizes that the last few hours may have been a hallucination. She calls the police, while Akiba recalls the events in Room 3 and realizes that Akai was actually the burn patient all along.
The narrative loops again, revealing the same sequence with roles reversed: Akiba becomes the burn patient who ordered the wrong drug, and Akai is cast in the role of Akiba, setting off the massacre of the staff. The hospital is evacuated the next day, with Nakazono surviving as the police search for Akiba, who has vanished. As Nakazono leaves, the red lights around the building flip to green and back again, and she cuts her hand, blood turning to green. A lingering shot shifts to the room where the burn victim was kept: a locker hums, green goo spilling from the seams, and, inside, a hand—Akiba’s hand, slick with goo—reaches out before itfalls to the floor.
