Kyou Kara Ma no Tsuku Jiyuugyou is the novel the anime and manga are based on. It's about a boy named Yuuri, who gets himself flushed down a toilet into another world. There, he finds out he's the Maoh (demon king) and that the Mazoku (demons) expect him to lead them against the human race. 
The novel is told from Yuuri's PoV, with plenty of side-commentary that is left out of the show.

Yuri Shibuya was living a pretty normal life. That changed the day he was dunked into a toilet after an attempt to save a classmate from a gang of bullies. Instead of just getting a good soaking, he's pulled in. The next thing he knows, he's in a world that vaguely resembles medieval Europe. If that's not odd enough, he's told that he is to be the next Maoh, just because he has black hair and black eyes. The Maoh is the King of the Mazoku, who are coexisting not-so-peacefully with the humans in this world. Much to his subjects' dismay, he's totally different from the rulers they're accustomed to. He's kind, considerate, a believer in justice, and not willing to use violence to solve conflicts. Not exactly someone they want running a country on the very brink of war. Now, Yuri has to deal with trying to become a good Maoh, while at the same time attempting to adapt to this lands' customs and culture, all in a world where the tension between the humans and Mazoku is reaching its peak.
 
(Source: Anime News Network)

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.

Life has returned to some form of normalcy after the end of the series, and so His Majesty Yuuri Shibuya travels to oversee how things are going. While doing so, he runs into the king of Small Shimeron--Sararegi. But it seems there may be something more to him than the friendly king he presents himself as. Yuuri go back to Shimaron to relax and meet some friends, when the "king" of Little Shimaron suddenly joins the group and wants the Maou to come to his country. The Original king sends a message to Yuuri's brother and his friend that Yuuri is in trouble.
 
(Source: Anime News Network)

Yuri Shibuya just can't catch a break. First, he had his head dunked in a toilet. Then he was transported to another world - where he finds himself king of the Mazoku's, a new race that expects him to lead them to war against all of humanity. And to top it all off, he's engaged - to a man!
 
(Source: Viz Media)

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