As a child, Miki Inoue, codenamed Imomushi, was taken by the Organization after her sister Mika's murder and trained as an assassin who kills only her assigned targets. Years later, hunting the truth behind Mika's death pulls her into a battle royale aboard the cruise ship Ageha, where a rogue Organization member has put a bounty on her head and turned her fellow assassins loose to collect it. This prequel spinoff of Arachnid is told from inside the Organization's ranks.