Plot

A writer moves to a remote island with his daughter and young son. After settling into their new home, a neighbor arrives to welcome them and give them a breakdown of the local rules; most important: do not go outside when the island's siren starts wailing.

The film begins by noting the sudden disappearances of people that occurred in the Lost Colony in Roanoke, Virginia in 1590 and the ship, the Mary Celeste, in 1872.

The film starts in 1976, the inhabitants of a solitary island of Yamajima seem to have dissapeared but searchers find one person who gives a mysterious warning: When you hear the siren, never go out.

Thirty years later, Yuki Amamoto goes to the island with her father, a freelance reporter, and her little brother, Hideo, who appears to be mute.

Though they are welcomed by a young doctor, Minamida, who shows them the island, all the islanders look creepy and unfriendly to Yuki. The house they live in has long been deserted, full of dust and old blood stains. Her neighbour, staying next door, helps her clean the house and gives her the same strange warning: When you hear the siren, never go out.

Yuki then talks to the doctor asking if Hideo will ever recover. He tells her it takes time. It is the beginning of a series of mysterious incidents, as Yuki explores the island: A group of dancers in fancy clothes, mysterious scribbles on the walls of a ruined house, a diary from 1976, a suddenly deserted market, a pretty girl in a red dress standing on the hill who appears to be telling her brother a story. Additionally, the diary is of the survivor of the 1976 annihilation of villagers, containing the truth of the incident. It describes how the murders took place, but the diary missing pages as it appears to have been torn in half.

After her father tells her he is going out to take pictures of the nocturnal animals, Yuki hears the siren for the first time. She is again reminded to not go outside but does so anyway. She goes looking for Hideo and finds him outside, pointing to the metal tower where the siren is coming from.

The next morning, Yuki and the doctor go looking for her father who has not yet returned. It is here that Yuki tells the doctor that the people of the village might not be trustworthy. However, Yuki then finds herself lost. She decides to explore the island more but runs across many odd occurrences. Eventually the doctor catches up with her. From the things Yuki has seen, she and the doctor summon a police officer to help them ash Yuki believes her father is dead. Nothing is found, but when she returns to her house, her father is there. But something is very wrong with him, as he seems cold-blooded and behaves like a different man. Curious as to what the problem is, Yuki finds her fathers video camera from the night the siren went off to see if there is anything on it. Nothing appears out of the ordinary with the exception of her father falling and slightly injuring himself. She discusses the island and the siren with the doctor again but returns home. She decides to find out more information on the internet. She reads about the Roanoke colony and the Mary Celeste. At the same time, the doctor begins to do some research on what Yuki has told her and, we see Hideo is being told a story by the woman in red again. The story appears to be about victims of a terrible disease were once isolated on Yamajima. The story goes that the flesh of a mermaid could bring ever-lasting life, a cure to the disease. They reportedly captured a mermaid and ate it. But before the mermaid died, it let out a terrible scream and cursed the island. Yuki sees old video footage from 1976 that was filmed by the rescue party who found the one survivor. After seeing this the website shows a picture of the woman in red.

Yuki rushes out to find Hideo and finds him sitting with the woman in red. Yuki tells Hideo to come to her and he does, just as the siren begins to wail again. She takes Hideo to an old house where a man is living and who had also previously warned Yuki of the siren. While there, Shibito begin attacking and attack the man tries to fight them off but is killed. This gives time for Yuki and Hideo to head back to their house. There, Yuki declares that she and Hideo will leave the island in the morning. While looking for things to take with them, Yuki finds a hidden room in their house. In the hidden room, there are pictures of people from 1976. Including a picture of the doctor who looks exactly the same as he did in 1976. Just as she discovers this, the siren begins wailing again and the power in the house goes out. Thats when Yuki sees that her father is now a Shibito. He tries to kill them but Yuki and Hideo are able to escape. Outside, they try to find help, but discover that every person on the island is now a shibito.

While trying to escape, Hideo points out the metal tower with the siren on it. Yuki and Hideo head that way and begin to climb to the top. On the way up, they are being followed by Shibito with many more coming to the metal tower. Yuki finally makes it to the top and begins to destroy the siren. After doing so, Yuki realizes she can still hear the siren. By then the doctor has climbed to the top of the tower, yelling to Yuki that there is no siren. Its all in her mind. And that Hideo died a long time ago. We then flash back to several scenes in the movie, seeing Yuki going through the same motions as before but Hideo is not there, explaining why people were looking at her oddly. We then flash back to a scene of her father and Yuki in an ambulance, with paramedics trying to revive Hideo but to no avail. Realizing that there is nothing left for her, Yuki lets herself fall off the metal tower.

We then see she survived the fall with the doctor and her father talking about Yuki and her recovery. While watching over her, the doctor notices Yuki still has the half of the torn diary in her pocket. He takes it from her and puts it together with the other half of the diary he had from the files of the massacre in 1976. Reading it, he sees that the person who wrote it states that when the siren sounded the fourth time, he must kill them all. We then see Yuki awaken to the sound of the siren. She appears behind the doctor with a scalpel and attacks. We then see the woman in red singing about hells gate opening as the movie ends with the sky turning black and the sea turning red.