"Hyakki - The Secret of Devil's Island" is a supernatural mystery anime set in a remote island prison where the boundaries between the living and the dead blur. The story centers on a group of inmates who are transported to a secluded facility known as "Devil's Island," a place shrouded in secrecy and rumored to be haunted. Upon arrival, the prisoners discover that the island is plagued by strange phenomena, including ghostly apparitions and unexplained deaths. As the inmates struggle to survive, they uncover the island's dark history, which involves forbidden experiments and a curse tied to the restless spirits of the dead.

The protagonist, a young man named Shun, becomes a key figure in unraveling the island's mysteries. Alongside other inmates, including a resourceful woman named Aya and a mysterious man named Kagetora, Shun investigates the supernatural occurrences. The group learns that the island was once the site of a government experiment aimed at harnessing the power of spirits, but the project went awry, resulting in the deaths of many and the creation of vengeful entities. As the inmates delve deeper, they encounter hostile spirits, cryptic clues, and the remnants of the failed experiment, all while being hunted by an unseen force.

The narrative explores themes of survival, guilt, and redemption, as each character confronts their past and the consequences of their actions. The island's curse manifests in various ways, forcing the group to rely on their wits and teamwork to navigate the dangers. The story builds tension through a series of escalating supernatural encounters, culminating in a confrontation with the source of the island's curse. The resolution ties together the fates of the characters and the island, revealing the true nature of the experiments and the spirits' motivations.

The anime blends elements of horror, mystery, and action, with a focus on character development and atmospheric storytelling. The main characters are defined by their personal struggles and growth, as they grapple with both external threats and internal conflicts. The plot is driven by a combination of suspenseful sequences and revelations about the island's history, creating a cohesive narrative that keeps viewers engaged. "Hyakki - The Secret of Devil's Island" offers a compelling exploration of supernatural themes within a confined and eerie setting, making it a notable entry in the genre.