Following the death of his parents, Shinpei Ajiro grew up with the Kofune sisters Ushio and Mio before heading to Tokyo to live alone. Two years later in July 2018, he returns to his hometown of Hitogashima Island, Wakayama Prefecture to attend Ushio's funeral following news of her drowning. However, Shinpei becomes suspicious when he receives news of strangle marks around Ushio's neck, implying that she was murdered intentionally. Now haunted by her "shadow" and assisted by Mio, Shinpei tries to find the answers to what really happened to Ushio and possibly save the residents from a strange dark enigma.
