Karasu Bank is among Japan's leading financial institutions, ranking third nationwide in operating profits. Akira Mitarai, a second-year employee at the bank's main branch, possesses exceptional mathematical abilities that allow him to instantly detect accounting discrepancies, but this talent also makes his routine work painfully dull.
One day, Akira receives an unexpected transfer to the bank's Special Operations Department, specifically to its "Appraisal Unit," which he is informed is a core division within the organization. Suspicious of both the transfer and the unfamiliar department, he learns from his new supervisor that the bank secretly operates a vast underground casino. The Appraisal Unit is responsible for overseeing this gambling enterprise and the gamblers who participate in it. However, these players stake more than just money—their very lives serve as collateral for the games. Higher risks bring greater rewards, but at the highest level, losing means death.
Overwhelmed and bewildered by the bizarre reality of the bank's hidden casino, Mitarai encounters a mysterious genius gambler named Shin Mafutsu. Captivated by Shin's talent for seeing through opponents' cheating and consistently winning, Akira decides to enter this world of life-or-death gambling as Shin's bank representative.