Each member of a band of assassins has a cover. The leader, Mondo Nakamura, is an officer of the law who bumbles through his assignments. The court sends him to investigate a mansion where a 300-year-old cannon of mammoth size is hidden, at the ready to defend the shogunate. Various factions have learned of the cannon and also converge on the mansion, which is guarded by an odd group of people whose ancestors have been the cannon's keepers. When an American gunrunner brings death to the innocent, Mondo and his comrades must spring into action. They face overwhelming odds, but there is honor in vengeance, even if they die in the fight.