Five demons launched a war against humanity, destroying more than half of the continent. To secure a temporary ceasefire, a political marriage is arranged between the most feared demon, the Demon King who has killed more humans than any other, and a duke's daughter named Cecilia. Cecilia accepts the mission to seduce and ensnare the Demon King, believing that by controlling him she can prevent humanity's extinction. However, when she meets her intended husband, she discovers that the Demon King is not a fearsome overlord but an innocent-looking beautiful girl. This unexpected revelation upends Cecilia's plans and forces her to navigate a strange relationship, caught between her duty to save humanity and the genuine, pure-hearted nature of the Demon King. The story unfolds as a fantasy action comedy with Girls Love undertones, following Cecilia's attempts to fulfill her mission while dealing with the complications of a marriage that is nothing like she anticipated.