The young prodigy Ichiro Umoregi, better known as Akuma-kun, possesses the ability to summon demons and appears once every 10,000 years. He tries to create a better world where humans and demons can coexist peacefully. Along with his demonic partner, Mephisto III, Ichiro uses his gift to run the Millenarianism Research Institute, which focuses on solving murder mysteries involving demons. During this process, Ichiro's personal search for his past results in the unleashing of a winged angel named Strophia, who has a history with Ichiro.