For generations, the Bernstein family has been recognized as a house of nobles who favor books over everything else. Elianna Bernstein, daughter of a baron of the Sauslind Kingdom, is no different. But instead of being called a bookworm, she is known as the Bibliophile Princess. Four years ago, the crown prince, Christopher Selkirk Asherald, offered Elianna a special arrangement: if she would become his fiancée, he promised that she could continue to spend all her free time reading. Despite having to inherit a title of great responsibility, she agreed to the proposal, enticed by the prospect of gaining access to the royal archives housing a wide selection of books. To this day, Elianna firmly believes that her engagement is void of romantic sentiment. When she notices Christopher's interactions with another girl, she presumes that the days she spends with him will soon reach an end. Yet, during this period of uncertainty, Elianna and Christopher open a new chapter in their relationship, gradually uncovering their genuine feelings as they grow to better understand each other.

After being stood up for a movie date, marketing employee Fumihiko Matsumaru is about to throw away his tickets when he is stopped by a girl who implores him to let her accompany him instead. Thanks to his upbeat and eccentric companion Aka Onda, an aspiring voice actress, Fumihiko enjoys his evening. While walking home together, they find out that they live in the same neighborhood. Mere hours later, Fumihiko wakes up from a nightmare and hears sirens outside his window. Going outside to check the situation, he sees that Aka's apartment has burned down, along with all her possessions. Fumihiko invites the distressed Aka to stay at his place, leading to them sleeping together. In the aftermath of that fateful night, their personal and professional lives become inextricably intertwined. Not only do they begin living together platonically despite their one-night stand, they also discover that Aka will be voicing the mascot Fumihiko designed for his company's newest product. While trying to keep their live-in relationship under wraps for fear of scrutiny, the two begin to support each other throughout the difficulties in their respective careers.

On his way home from school, Tatsumi sees a man collapsed near a lake. When he approaches him, Tatsumi notices something strange: the person in need of help is actually a beautiful merman named Wakasa! Because Wakasa's home has become too polluted to live in, Tatsumi graciously offers his bathtub as a refuge. With a boisterous merman as his new roommate, Tatsumi's normal life won't be returning anytime soon, not to mention Wakasa's aquatic friends—Takasu, Mikuni, and Maki—often show up uninvited, making them all quite a handful for the high school student. As he humors their curiosity for human life, Tatsumi sometimes finds himself enjoying their childish antics, but he will have to keep his cool if he intends to keep up with his daily life and newfound friendship.

At the end of season 2. Yuuri defeats Shinou, but since Shinou had been defeated, Yuuri and Murata could no longer go to the Shin Makoku. However, because Yuuri had fought and defeated him, his powers had surpassed that of Shinou's, resulting in him being able to rely on his own power to return to Shin Makoku. After the ceremony at the age of 16, Yuuri slowly begins to discover that much has happened while he was away. The ten officials of the noble family have decided to make Wolfram the next Maou, but now that Yuuri has returned and taken back his role as Maou, complications arise when one noble disagrees with Yuuri returning to the throne. In addition, an ancient secret society that threatens the peace of both humans and Mazoku has risen, and a fight for the throne that now leaves the nobles wondering who is the legitimate Maou.