"Fūto Tantei: Kamen Rider Skull no Shōzō" is a spin-off of the "Kamen Rider W" series, set in the fictional city of Fūto. The story centers on Sokichi Narumi, a private detective who transforms into Kamen Rider Skull, and his partner, Akiko Narumi. The narrative unfolds as Sokichi investigates a series of mysterious incidents involving the use of Gaia Memories, dangerous devices that grant users superhuman abilities but often corrupt their minds. These incidents are linked to the criminal organization Museum, which produces and distributes Gaia Memories.

The plot begins with Sokichi and Akiko taking on a case involving a young woman named Kiriko, who is targeted by Museum operatives. As the investigation progresses, Sokichi uncovers deeper connections between Kiriko, Museum, and the origins of Gaia Memories. The story delves into themes of justice, sacrifice, and the consequences of wielding power, as Sokichi confronts his own past and the moral dilemmas tied to his role as Kamen Rider Skull.

Key characters include Sokichi Narumi, a seasoned detective with a strong sense of justice and a tragic backstory tied to the death of his wife. Akiko Narumi, his spirited and resourceful partner, provides both emotional support and comic relief. Kiriko, the central figure in the case, becomes a catalyst for uncovering the truth about Museum’s operations. The antagonists, primarily members of Museum, are portrayed as ruthless individuals driven by greed and ambition, with their actions posing a significant threat to Fūto.

The series maintains a balance between action-packed sequences and character-driven storytelling, exploring the relationships between Sokichi, Akiko, and Kiriko while advancing the overarching mystery. The narrative builds toward a climactic confrontation between Kamen Rider Skull and Museum’s leaders, culminating in a resolution that ties together the personal and thematic threads of the story. The series serves as both a standalone entry and a prequel to "Kamen Rider W," offering insight into the events and characters that shaped the larger narrative of the franchise.