A woman called Hashimoto is in a car accident and awakes in hospital. Sugie, the driver of the other car, is in the next bed and angrily demands that Hashimoto pay compensation. Hashimoto notices a stinging sensation around her lips and mouth, which the nurse tells her must be an effect of the crash.
When Hashimoto goes to sleep that night, she is awoken by someone talking. She soon realizes that it is coming from the other four patients in the room, even though they appear to be asleep. They all rise from their beds in a trance-like state and file out of the room. Sugie continues sleeping soundly.
In the morning, the doctor is worried about the four patients, who are refusing to eat. They also won't leave the hospital because they want to remain together rather than be treated as individuals. Sugie asks to be moved from the ward since she doesn't want to share a room with Hashimoto. She wants to move into Ward One, which is empty; but discovers that it has been turned into a storage room due to the hospital's lack of patients. The four women encourage her to return to Ward Three with them.
Sugie and Hashimoto continue to argue over Sugie's demands for compensation. Sugie notices that her face has begun to sting, and thinks she is catching a virus from Hashimoto. She moves into Ward One on her own. After she leaves, the stinging in Hashimoto's face stops. Hashimoto awakes that night to find the four patients are missing. She thinks it strange that they do everything together, even going to the toilet together, and claim to have the same dreams. Suddenly, she is startled by a strange old man called Mr. Furuta. He says that he is in Ward Two, which is the men's ward next door. He has broken a hole in the wall so he can pass between the rooms. The nurse comes to take him back to his bed.
Sugie begins screaming and rushes out into the hallway. The four patients are attacking her with long, sharp, snake-like tentacles coming from their mouths. The hospital staff can't see anything but Hashimoto does. She realizes that the stinging in her face was caused by the patients targeting her and piercing her cheeks with their tentacles, but she just couldn't see them before. The doctor and nurse discuss the four patients' mysterious case; there were initially just two of them, who were admitted to hospital because they refused to eat. New people who come into the hospital are frequently scared of them. The doctor and nurse agree to stop feeding the patients via intravenous drip, unaware that the patients are eavesdropping outside.
The patients, now joined by Sugie, burst into the room and kill the doctor and nurse by using the tentacles to fire sharp needles at them. Hashimoto deduces that some kind of creature is using multiple people as its host and the tendrils must be its nervous system. She tries to escape via the hole in the wall but the patients soon discover where she is. When one of them corners her, she uses a scalpel to sever the tendrils coming from the woman's mouth. The host body collapses to the floor and dies, giving Hashimoto time to escape from the others. She hears the creature taunting her to run and saying that it will find her because Sugie had noted the licence plate of Hashimoto's car.