Shinpei Ajiro returns to his hometown of Hitogashima Island after two years to attend the funeral of his childhood friend Ushio Kofune.

While reuniting with Ushio's younger sister Mio and best friend Sou, he learns about Ushio's mysterious cause of death and the legend of the island "Shadow Disease"

The legend of the shadows is that those who encounter a shadow that has the same appearance as themselves will die.

The shadow of death haunts Shinpei and the others. And a day of despair arrives on Hitogashima.

"How can I save my friends, how can I protect everyone on the island, how can I avoid death..."

In order to avoid despair, Shinpei is constantly forced to think and make a choice, but each choice may be nothing but to open a new door of despair.

However, he cannot stop thinking and making a decision to overcome the three days of despair.