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.
Big Cimaron has taken control of Francia making escape hard for Yuri and the others. Conrad forces Jozak off a cliff as an answer to the question of his betrayal. When King Belar shows up Conrad is told to execute Yuri.
Francia has made Conrad a prisoner and decided to join in alliance with the Demon Kingdom. When Conrad escapes from his cell Yuri and the others must chase him down and hopefully find out his reasons for his betrayal. Can Conrad be forgiven and if he is will he be able to leave Francia with his life?
Yuri's older brother Shori reminisces about a time in Yuri's youth when he would call him his "big brother." When things look bad for the Shibuya parents, Shori takes his little brother away for a day at the amusement park. Find out what happens when little Yuri accidentally spills his ice cream on a thugs pants.
Baby Ernst, Eru for short, the son of Hube, and several other noble born children are gathered at Covenant Castle for the "Great Birth Ceremony." After Yuri volunteers to look after baby Eru he finds out that taking care of a baby is harder than he expected especially when Eru turns out to be "special." What happens when Eru gets his hands on Anisina's latest invention, a device that allows the wearer to talk with animals?
A mysterious girl who bears a strong resemblance to Ulrike is seen being mischievous around town. Meanwhile back in the Tomb of the Great One Ulrike is sick and unable to perform her normal tasks. When it turns out that the mystery girl has amazing magic powers it's up to Yuri, Conrad and Wolfram to get her under control.
Stoffel has plans to set up Yuri with an appropriate princess thereby cementing his ties with the current ruler of the kingdom. When Princess Elizabeth, the current winner of the Demon Kingdoms beauty pageant, shows up Yuri is faced with a battle between his current fiancé and the new princess. Will Yuri's devotions be transferred to this new princess or will this princess reveal an entirely different agenda?
Yuri and co. have gone through lots of boxes in order to find the last box but coming up empty. They learn of a box in a lake and go to investigate. There they meet a girl, who turns out to be Urike's predecessor. They find out that this lake and land were barren before the arrival of the box. If they remove the box the lake and the land will die. Yuri wants to leave it and just guard it but when Big Cimaron shows up to take it not knowing or caring if it's the real thing or not. Can they stop them and save the lake and the land or must it be sacrificed?
Dunheely, Conrad's father, has trouble getting along with Gwendal especially when Gwendal despises him. When Dunheely has Gwendal join him in helping a secluded town of outcasts Gwendal must learn to see his step-father as the person he truly is. Will Gwendal's prejudices get in the way of saving the mistreated villagers?
After having their box stolen by the Maou, Big Cimaron finds a new way to impose power over the allied countries: an "overseas study exchange". However, The Great Demon Kingdom and the allied towns are not stupid. In order to put an end to Belal's ambitions, Gwendal offers himself to go negotiate with him on Van da Via island, hiding his true intentions to the Maou.
Adalbert has taken King Yuri hostage and demands to know if Yuri does in fact have Julia's soul. Big Cimaron's troops try to capture Yuri prompting Adalbert to try and save him. They are caught against the side of a cliff with an active volcano as a backdrop. When the volcano decides to erupt will everyone be able to leave the island safely?
Panic consumes the troops of both nations and Big Cimaron's king decides to leave the children he already has in order to escape the volcano's fury. Adalbert decides to fight Conrad now and unleash all of his pent up sadness and fury over Julia's death. Will the children be saved and will Conrad be able to escape from Adalbert's emotions?
Maxine still has not given up hope of returning to his country and becoming wealthy and powerful. When he kidnaps Yuri and tries to escape he runs into one of Yuri's devoted subjects and is forced to abandon his plan. In his desperation he decides to hold the three maids as hostages. Will he make it out of this alive or will he be forced to take even more drastic measures?
When Yuri stumbles upon the "Demon Mirror" he's transported back to the time when Lady Celi ruled with the help of Stoffel. Yuri winds up meeting Lady Julia and comes to understand why she was loved by so many people. Yuri and Julia must watch as Conrad and the Luttenburg Division set out towards a suicidal battle in order to prove their loyalty to the kingdom. Yuri helps Julia to make a decision that she knows will hurt the one she loves most dearly.
Günter sets off on a "Nation Wide Tour" in order to better understand the "Common People." He comes to a town modeled after the old west and discovers that the town is full of fear and that only one person holding it together is a girl by the name of Gloria. Can Günter help this forgotten town survive and can he convince the people that the Demon King has not forgotten them?
A man cloaked in shadows shows up wielding the power of the holy sword. Yuri's followers suspect that Alford is behind a series of bandit attacks where the holy sword had been spotted. Who is the strange cloaked man? Why does he change people into heartless bandits?
Yuri and the others go to check out the rumors about the Holy Sword and stumble upon a man possessed with demonic energy who wields the Holy Sword. Alford shows up in search of his sword and winds up helping Yuri and the others retrieve it. It might take Morgif's full powers to defeat the new owner of the Holy Sword. Will Yuri be able to handle Morgif's new powers and why does Murata sense that they are running out of time?
A mysterious masked man with hypnotizing powers walks into King Belar's castle unhindered leaving us with many questions yet to be answered. Back in the Demon Kingdom Yuri is holding a party for the Alliance members. When Conrad's left arm and Gwendal's left eye begin to hurt they can not help but have a sense of looming disaster. Faceless puppets attack Yuri at the party and run off with Yuri's protector. Yuri follows them without thinking and rushes off to save his friend.
Yuri is captured along with Wolfram and taken to Big Cimarion in order to be handed over to a hypnotized King Belar. Their transport is attacked by bandits who are led by a familiar face. The two of them are taken to the bandit's hideout where they find old enemies who have become reasonably good people. The masked figure sends a giant blue dog-like demon and several small dog-like demons to attack the hideout and retrieve Wolfram and Yuri. Yuri's power, in the form of water dragons, is little match for the power of the unusual beast master. Will Yuri be able to overcome these strange new foes and can anyone help him?
Yuri decides to confront King Belar and try to convince him to stop fighting. He goes off with his crew and Adalbert in tow towards the Kings Castle. After many confrontations with more puppets Adalbert tells Conrad and Wolfram to take Yuri to Belar while they finished off the puppets. When Yuri arrives in the kings throne room he learns that King Belar is being controlled also. When they are attacked by allies it's up to Yuri to use all of his skills and resources to save them all. Who is this enemy that has knowledge from the time of the Original King and where is the final missing forbidden box?
Ulrike reveals that the final forbidden box is on Earth. Yuri, along with Murata, Wolfram, Conrad and Gwendal set off to locate and bring the box back to The Demon Kingdom. Günter was also dragged along by mistake. Yuri is shocked to discover that his family already knew about the other world the whole time. Meanwhile, Günter was separated from the rest of the group and is lost somewhere in the city.
Yuri takes the group on a sightseeing trip while waiting for Bob, the Maou of the Earth to contact them. Shori meets up with Rodriguez, who ask a favour from him. He asks Shori to persuade Conrad and the others to return to their own world. After seeing Shori's failed attempts, Rodriguez attacks Conrad with magic and tries to force him back to the other world. Why is Rodriguez doing this? And is he doing this on his own or following the orders of someone else?
Yuri and co. travel to Switzerland after discovering that Bob is currently staying there. Yuri is shocked when he finds his parents followed him there and they travel together to meet Bob on a bus that was sent to fetch them. When they reach their destination, Shori separates Yuri from the rest of the group and once again tries to persuade him to stop being the demon king and come home but he is interrupted. They then continue on to Bob's office, ignoring his warnings. Meanwhile, Günter, who was working as a model to earn money, is taken away during a blackout at a photo shoot.
Yuri and Bob battle with magic and Yuri wins. Bob decides to entrust the final forbidden box to Yuri.
The bone tribe is acting up due to the fear of all of the forbidden box's are gathered together.
Günter is poisoned again and Suzanna Julia's retired father visits the Demon Kingdom.
It seems that one of the maids has been affected by miasma, a disease that makes the person unable to trust anyone. Yuri, Wolfram, Jozak and Conrad must obtain a rare herb to cure the maid but how will they succeed if they are affected by it too?
Yuri along with Conrad, Murata, and Wolfram travel back to earth in order to get something Yuri left behind. What happens when they can not return to the other world?
Human villagers decide to build a bridge between humans and Mazoku. But will the bridge be destroyed by some mysterious light wolves or will there finally be the peace that Yuri has been hoping for?
Günter teaches Yuri about the past of The Great Demon Kingdom.
Shori goes to The Great Demon Kingdom.
Wolfram offers to give Shori magical power to protect Yuri but before Shori can answer, an unexpected visitor arrives interrupting the conversation. As Ondine, Ulrike's predecessor, lays unconscious a storm rages on. Meanwhile, mysterious figures attack the castle. It appears that Seraphine, Ondine's older twin sister, was filled with rage when she was not chosen to become the next Genshin Miko and left her sister. Now, she is back and believes it is her job to destroy the keys to the four forbidden boxes. Shori is worried for Yuri and accepts Wolfram's offer. He finds himself now being able to control water. Back on Earth, Seraphine and her comrades attack Bob and flee after realizing that he is not a key.
Shori, while attempting to protect Yuri, runs wild and loses control of his powers.
Yuri and co. head over to The Original King's Temple so that Yuri can carry out the ceremony to seal the four forbidden boxes.
Yuri starts the ceremony but Wolfram suddenly attacks Yuri, disrupting the ceremony. A dark figure appears and takes Conrad's arm, Gwendal's eye and Wolfram's heart as the keys to open the boxes. It absorbs the power from the boxes, revealing the form of The Original King! Fortunately, Jozak jumped in in time and Murata activated a trap, imprisoning The Original King.
Yuri and co. return to Covenant Castle to form a plan to defeat The Original King. Murata reveals that Soushu had possessed The Original King.
Yuri and co. head back to The Original King's Temple to retrieve the keys from The Original King. However, they are attacked by Soushu's army of dolls when they enter. Yuri forces his way through to Soushu, who escapes the barrier. He returns the keys to their owners, as they are no longer needed. Murata then pushes Yuri towards Soushu, allowing him to be captured while the rest are transported outside.
Soushu enters Yuri's body to take it over from the inside. Meanwhile, Conrad, Gwendal, Jozak, Hube and Günter awaken to find themselves outside the temple. They return to the castle, where Wolfram had been revived.
Soushu has successfully taken over Yuri's body and his army begins attacking countries everywhere. Meanwhile, Günter and Dorcacos travel to Morgif's birthplace in order for him to reveal his true powers. The rest of the group heads over to The Original King's Temple to save Yuri but are shocked by what they find there. The Original King tries to use Yuri's blood as the key to the fourth box, but Julia's pendant reacts to the act and transmits a vision of Julia into Yuri's soul, where she tells him that he is strong and can save everyone. Just then, Günter and Dorcascos arrive with Morgif.
Soushu is forced out of Yuri's body and Yuri defeats him, causing Soushu's troops to disappear. Murata told Yuri that they can not travel again to the other world because The Original King had disappeared. The Original King opened one last portal and Yuri, Murata, and Shori decided to go back to Earth. On Earth, Yuri worries about everyone in the other world. When he takes his necklace close to the lake it shines, so Yuri begins to realize something. Then Murata suddenly pushes him into the lake and they arrive in the other world.