Sōtarō investigates mass disappearances wreathed in a metropolis's purple smoke as a detective for the Ootsuji Agency. His pursuit descends into Kuragari, a dreamlike subterranean realm beneath Ogi-town, its labyrinthine roads teeming with bandits and shadowy elements. During this descent, he encounters the armored train Sokore 463, commanded by Tanne and armed with a 75mm mountain gun. Sōtarō is repeatedly rescued by Tanne from perilous situations.

His investigation reveals complexities far beyond the initial vanishings, encountering laughing mask gangs, conductors, and underground police forces. The mystery expands vastly in scale and implication. Sōtarō's story unfolds in medias res, creating an unresolved atmosphere despite significant events. His journey alternates between physical descent and potential exploration of deeper consciousness, with memories or dreams manifesting visually as puppet-like sequences. No concrete information exists regarding his background, personal history, or development beyond these events.