Over a decade before the events of the manga series, a chance encounter leads professional bodyguard Yukichi Fukuzawa to begrudgingly take an arrogant boy genius named Ranpo Edogawa under his wing. Their first job brings them to a local theater on high alert after receiving a bizarre death threat — a challenge that will require both Ranpo's brains and Fukuzawa's experience. The aftermath spurs Fukuzawa to establish the Armed Detective Agency, an organization that takes on dangerous jobs even the police won't handle!