684 AD - more than 1,300 years in the past. That year, a terrible famine struck a village, its people starving to death one by one. And then, when it seemed as though the village would be wiped out, a comet appeared in the sky, and along with it came Datatsushi (a being from the Underworld - another lifeform influenced by Annaki). To the humans, Datatsushi appeared to be a "god" that had come down from heaven, an absolute being that should be revered and worshipped. However, despite intuitively knowing that it was forbidden, the villagers - on the verge of starvation - ate the "god". By eating its raw flesh two male villagers and a pregnant village girl luckily avoid starving to death, but when the "god" lets out its dying cries those who committed the forbidden action are cursed by the being that is superior to humanity.
Hisako Yao, the village girl who becomes a part of the god by consuming Datatsushi's flesh, is granted eternal life, but this also means that her descendants, too, will be cursed forever. Hisako begs for the god's forgiveness by offering up her own flesh and blood (descendants), but eventually her own memories and personality are lost in the eddy of fate, and after many years the ritual itself becomes her goal.
In the present day (2003), Hisako offers up Miyako Kajiro, her own descendant, but Miyako rejects her fate and causes the ritual to fail, turning the village into the Other World. Hisako's biggest surprise - and mistake - is Kyoya Suda, a boy who is one of the humans who becomes trapped within the Other World, who obtains Miyako's power and wields the Homuranagi and Uryen, divine weapons that are the only way to defeat Datatsushi. Hisako fights back by giving Datatsushi her own flesh and blood to "complete" it, but is defeated by Kyoya and Miyako's strength and falls into a dimensional abyss. Kyoya and Miyako, equal to the god, send Harumi Yomoda, a young girl who is the only one not to have taken in any red water, back to the real world, bringing an end to the three-day-long story.
Ouroboros - a loop in spacetime
The Other World, where Datatsushi lives, is on the border between the real world and the Underworld, and remembers the curse that was first cast. Each time the ritual fails, land and people from various ages in the real world become trapped there, but since the dimensions are warped all along, time and space in the Other World are a vague concept.
The siren
In the Other World, Datatsushi sounds the siren - a call for those who have made preparations for becoming closer to beings of the Underworld to head into the red sea - once every six hours, four times a day in total. Those who go to the red sea even once are no longer human, ultimately becoming a part of the red sea.
Visions
The ability to intentionally steal a glimpse or sound of another's sight or hearing. Datatsushi (as well as the Kajiros) and the Shibito use this as a method of communication, and humans who have taken in the red water gaining this ability is a sign that they are turning into a Shibito.
Umi-okuri/Umi-gaeri
People who submerge themselves even once in the Other World's red sea are no longer human, turning into Shibito (beings of the Underworld). This phenomenon, as it is performed in the Other World, appears as though it can be seen across all time periods due to dimensional instability, recorded by those who witnessed it as a "legend".
Shibito
According to the amount of red water they have taken in, Shibito mutate into various forms over a period of stages. The first stage is "half-Shibito", but as it progresses they turn into animalistic forms like dogs, spiders and dragonflies, a far cry from their human appearances. Some special exceptions, like the leader Brain Shibito, also exist.
A world of truths and falsehoods
The various subplots in Siren do not end with the game's events and archives. By investigating the various online content and putting it all together, you can decipher the full picture. These gimmicks were also carried over into the sequel, Siren 2.
Kyoya Suda, resurrected by the power of the red water after being shot in the chest at the very start of the strange happenings, continues fleeing with Miyako after meeting her by chance. Miyako, touched by Kyoya's kind-heartedness, is unable to sit by and watch as he turns into a half Shibito, and tries to save him by mixing her "fruit" (blood) with his. This is because Miyako's blood - the cursed blood of the Kajiro family - has the power to prevent the transformation into a Shibito (cursed not to get close to the god), as well as endlessly use the Uryen without giving up one's own life (the curse of eternal life). Afterwards, Miyako is captured by Hisako Yao and her corporeal form destroyed. However, she remains connected to the depths of Kyoya's mind after having shared her blood with him, and supports him by becoming his eyes and mind. In order to fulfil his promise with Miyako (to end the Kajiro family's fate), he directly confronts the source of the curse.
Reiko Takato, a teacher at Hanuda Village Elementary School Oribe campus, and her student Harumi Yomoda become caught up in the strange events while out on a school trip. Takato, traumatised by the loss of her beloved daughter, views Harumi like her late daughter, and decides to protect her with her life in the Other World. Seeing that Harumi is about to be captured by a Shibito, Takato uses herself as a decoy and blows herself up along with the Shibito. Her noble act of self-sacrifice saves Harumi, but ultimately she becomes a half Shibito and attacks Harumi. However later, after becoming a Shibito Brain, Takato saves Harumi in a desperate situation when she is attacked by Principal Nagoshi at the climax of the story. Even after becoming a Shibito, her desire to protect Harumi remains strong.
Tamon Takeuchi, a folklorists with strange abilities who develops unique theories through his sharp sense of intuition and thorough investigation, obtains information that the "secret ritual" is to be performed, and decides to visit Hanuda Village to conduct an investigation. Takeuchi, albeit faintly, realises the truth behind the odd goings-on, and ponders a way to escape from the Other World, as well as stop the cause of the incident, from the standpoint of a folklorist (he must also have had feelings for his late parents and hometown). Along with Yoriko Anno, his student and assistant who makes him take her along, he visits the village and begins to investigate, but they both take in red water and are no longer able to return to the real world. The two have also received some of Kyoya Suda's blood, so they will not turn into Shibito. The two continue their investigation into a way to return to reality.
Twin brothers, born into the Yoshimura family, lost their parents in the Hanuda Village landslide 27 years ago, and were raised as strangers, taken in as successors to the hospital and church. The elder (Makino) has the expectations of all of the villagers placed upon him as Guiding Priest, the younger (Miyata) becoming director of the hospital, but forced to take part in corrupt acts behind the scenes. The two, each with complicated feelings toward the other, quickly get closer after becoming trapped in the Other World. Seeing Makino in a deep despair after the failure of the ritual, Miyata's previously held ambivalence spills out, taking on the role in place of his brother (by killing the "younger brother" that he is). This "true Guiding Priest", fusing into one being, gives his life to save the people of the Other World.