Atsushi's far-fetched idea to form an alliance with the Mafia results in a summit between crime boss Ougai Mori and Armed Detective president Yukichi Fukuzawa. But when negotiations fail, Dazai has no choice but to infiltrate enemy territory alone to secure Q. There, Dazai runs into some unexpected trouble...until a familiar man appears before him...!

The entirety of the majestic Sky Casino is turned against the Hunting Dogs when they uncover a terror plot that will shake the very foundations of the world. Their only hope is to capture the casino's general manager, Sigma, the man without a past! Meanwhile, Atsushi and Kyouka fight from the shadows, seeking to prove the innocence of the detective agency through their actions. Can they find the all-powerful "Page" that's at the core of the Decay of the Angel's villainous plot?