Atsushi Nakajima and the Armed Detective Agency open Season 4 in crisis: the Decay of the Angel — a terrorist ring run by Fyodor Dostoevsky and Nikolai Gogol — has framed them for a string of murders. A separate flashback arc traces how Yukichi Fukuzawa and a young Ranpo Edogawa crossed paths on a case that led to the Agency's founding, placing the current siege in historical context.