Aki is an 18-year-old salvager and mercenary residing in the impoverished Bottom, the lowest socioeconomic layer beneath a stratified society dominated by the advanced upper city Top. He operates a humanoid combat vehicle called an AT (Altro) to navigate underground tunnels, scavenging wrecked military equipment—primarily decommissioned AT units from the Hundred Year War—and dismantling them for salvageable parts. This work often forces him to defend against hostile subterranean creatures.

Despite his exceptional AT piloting skills utilized for salvage and combat, Aki feels profound dissatisfaction with his life in the valley. He dreams constantly of ascending to the technologically advanced Top, an aspiration driving his desire to escape his circumstances for a more significant existence.

His routine shifts dramatically when Deerhalt, an envoy from Top, recruits him for a mission: rescue Cendrillon, the kidnapped daughter of a Top chairman. Aki idealizes this task as a fairy-tale narrative where he, the heroic prince, saves the princess. This romanticized perspective fuels his enthusiastic investigation of the abandoned structure "the island."

During the mission, Aki demonstrates his combat prowess in confined tunnels and derelict buildings. He navigates physical challenges and opposition tied to the kidnapping with technical skill interwoven with persistent naivete, maintaining his romantic view throughout this primary documented experience.