Ryuuichi Sudo, a college student nearing his final year, accepts free tickets from his friend Seigu Takashi for a summer mystery tour. Accompanied by his childhood friend Wakaba, he arrives at Oka Island—an isolated, post-apocalyptic wasteland abandoned eight years earlier following coal mine collapses. Ryuuichi’s inquisitive nature and playful mischievousness contrast his underlying kindness as he interacts with fellow travelers Suzuki Hiromi and Inagi Shouko, exploring the island’s crumbling infrastructure under the watch of its enigmatic owner, Mr. Okouchi.

Okouchi’s orchestrated horrors escalate, pushing the group into psychological turmoil. Ryuuichi delves into Oka’s ruins, piecing together its tragic past and confronting spectral manifestations tied to the island’s "100 demons" lore. His discoveries unravel the curse, triggering a climactic supernatural upheaval that revitalizes the desolate landscape. Though normalcy seemingly returns, a post-credits scene obscures Okouchi’s ultimate fate, while subtle cues imply Ryuuichi’s escape from the island’s lingering shadows.