An incident occurred on Rougetsu Island in which the islanders mysteriously vanished.
The cause of the horrific incident was the "Kiraigou", a festival that welcomes back the dead.
The story of its resurrection has been briefly summarised.
Please use this as a reference for figuring out the story.
The Rougetsu Kagura was held once every ten years on the night of the lunar eclipse.
A shrine maiden called the "Utsuwa" (vessel), five young girls as musicians known as "Kanade" (instrumentalists) and the audience would all wear masks while participating, as standard for this kagura.
The maiden dances, the pace of the dance escalating, ecstasy, and then a trance...
The islanders appear to have treasured that moment, which came once a decade.
On the other hand, the islanders feared the kagura due to its connection with an illness called "Getsuyuu Syndrome".
Getsuyuu Syndrome, an endemic found only in the Rougesu archipelago, is a disease that causes memory loss.
As the illness progresses, when a person looks in the mirror their face appears warped, and they display delirium as their personality changes, until they develop an extreme fear of looking into mirrors.
These symptoms were known as "Budding" on the island.
In the Budding stage, other people cannot see the blurring of the face.
However, as the symptoms worsen and the person "Blooms", other people can apparently also see the warping.
When a person looks at the face of someone who has Bloomed, they too Bloom.
Blooming was the islanders' greatest fear.
Moon Wandering (Possessed Wandering)
People infected with Getsuyuu Syndrome wander around, obsessed with the moon.
Moonlight appears to be imbued with a strange power that awakens memories, and by continually bathing in moonlight patients could attain a state of mental equilibrium.
However, many patients tended to commit suicide when they regained self-awareness.
In order to prevent themselves from totally breaking down...
The wife of Shigeto Haibara, director of Haibara Hospital, was one of these people.
Modern medicine and the festival master's lineage
While ensuring the continuation of the family line of the festival master, Shigeto Haibara also searched for a cure for Getsuyuu Syndrome from a medical aspect, becoming involved with illegal treatment methods along with his son, You, after the death of his wife.
This was for his daughter, Sakuya, who began showing signs of Getsuyuu Syndrome perhaps due to having served the role of shrine maiden in the Rougetsu Kagura twenty years before Ruka and the others visit Rougetsu Island.
Musical therapy based on the "Lunar Melody", something passed down on the island since ancient times.
Treatment using a strange device, brain surgeries...
However, none of these various experimental treatments helped to return Sakuya to normal.
In the end, this led to the kagura being performed again, which caused an onset of the illness.
The Rougetsu Kagura's true form, the forbidden Kiraigou, was also performed...
The Yomotsuki family is a family of craftsmen who create the masks used in the kagura.
Its founder, Souan, is said to have created a mask in which the souls of the dead dwelled.
Seven generations later, Souetsu, famed as the one who could create masks out of death and souls themselves, was born.
However, Souetsu's mask-making techniques ceased use in the Kiraigou when it brought about a disaster called the "Day Without Suffering".
Since then, the Kiraigou festival has been forbidden, and turned into the Rougetsu Kagura for tourists.
Souya Yomotsuki, who became the final craftsman of the Yomotsuki family, was shown a photograph by Shigeto Haibara that was said to have been taken by Dr. Asou.
It showed the Mask of the Lunar Eclipse made by Souetsu, the jet-black mask that brought the Day Without Suffering to the island.
He was entranced by it, and shut himself away in his workshop in the Hall of the Conscious Mind, putting the masks he made on his daughter, Ruka...
This is how he spent his days.
And then finally, Ruka began to be attracted by the moon.
Path to the Lunar Underworld Hall
Dr. Asou's camera that can photograph the past, the Camera Obscura, also revealed the key to the door enclosing the entrance to the forbidden stage of the Kiraigou - the sheet music to the Lunar Melody, lost long ago, which was restored from the burnt remains of old documents left to the Haibaras.
The two kaguras
In the Lunar Eclipse Hall the for-tourists Rougetsu Kagura was performed, using Tsubaki Tono as the Utsuwa...
In the Lunar Underworld Hall, the ancient Kiraigou was performed, using Sakuya as the Utsuwa...
The five spirited away girls - Ruka, Misaki, Madoka, Marie and Tomoe - were the Kanade.
However, the Kiraigou failed, and Sakuya became a being that was dead although living, the five girls losing all memory of the time before the ritual.
The Moon Reflection Cave/Moon Reflection Well
Two weeks after the kaguras, the five spirited away girls were rescued by a detective called Kirishima.
They were standing under the Moon Reflection Well - it was said in books that by standing under and bathing in moonlight that shone down through it one could recover their lost soul...
Fortunately, they were able to escape the vanishing islanders incident that occurred two years later by leaving the island.
On Rougetsu Island a strange custom was practised in which the faces of the dead who had Budded were cut off.
This was because faceless people could not Bloom.
However, it was strictly forbidden to cut off the faces of the living.
Due to this, the face-cutting was never performed on Sakuya, who was still living although she was dead, in the Mourning Shrine in Rougetsu Hall's basement.
The Day Without Suffering
Long ago, in Souetsu's time, the Utsuwa of a failed Kiraigou Bloomed, bringing about the Day Without Suffering.
Second Coming of the Day Without Suffering
Eight years before, Sakuya awoke in the Mourning Shrine and wandered the island, gradually causing those who saw her face to Bloom.
There is a legend that those who fled to the tunnels in Souetsu's time survived, but after the second failed kagura Sakuya's Blooming was even more severe, so that even those who escaped to the tunnels were not spared and also Bloomed.
Souya Yomotsuki, who waited for the end in the Meditation Cave, was no exception.
This is how Rougetsu Island came to be abandoned, becoming taboo.
If a third kagura was performed Sakuya would Bloom even worse, causing huge Resonance...
This would engulf the island, and spread the disaster to other islands.
However, the only way to seal the Bloomed Sakuya was to put the same Mask of the Lunar Eclipse on her and perform the Kiraigou, to return the souls to the moon...
Including the thing the second Kiraigou was lacking...
The thing that was missing... It was the sound of the soul, the Tsukimori Song.
What fascinated Souya about the Mask of the Lunar Eclipse was the thought that with the true Mask of the Lunar Eclipse he could reach the Zero Stage, the souls' source.
He began to make masks as though possessed, forsaking his family.
Souya never knew that he was within touching distance of the Zero Stage that he so yearned for...
The Tsukimori Song, resonating with the souls and bringing them peace - it was passed from Sayaka to Ruka, and in order to complete the third Kiraigou it had to be played at the closest place to the moon, the Gessou Shrine.