Yuri Shibuya was just a normal Japanese high school kid on his way home from school he sees a former classmate of his, Murata, being bullied. He steps in to try and stop the bullies but they just grab him instead and dunk his head in a toilet in the girls' bathroom. Yuri is sucked into the toilet and ends up in another world. When he wakes in an unfamiliar area, Adalbert von Grantz unlocks the local language hidden in Yuri's soul. Conrad Weller then rides up and after a short fight Adalbert runs away. Conrad takes Yuri to a small village where he meets Günter von Christ who tells him that he is the new king of the Demon Kingdom. They then take Yuri to the castle where he meets Wolfram von Bielefelt and Gwendal von Voltaire, Conrad's two half-brothers (who do not look anything alike!).

While attending a dinner party that the former Queen is holding, Wolfram insults Yuri's mother by saying she is dirty human and Yuri reacts to the insult by slapping Wolfram across the left cheek. From everyone's weird reaction to the slap he finds out that this is actually how the nobles of the Demon Kingdom propose. Wolfram then throws his silverware on the floor to challenge Yuri to a duel to contest the engagement and Yuri unknowingly accepts by picking up a knife. Yuri is allowed to choose how they fight the duel and so he decides they should fight through a sumo match.

When Yuri wakes up after his first time using magic he finds out from Wolfram that Conrad and Gwendal have gone off to protect a human refugee village that is being attacked. Yuri convinces Wolfram to take him out to help. On the way they run into Adalbert who tells Yuri that it is humans that are attacking the town so it is a fight between humans. He tries to convince Yuri to come with him and leave the demons but Yuri decides he is going to help the demons. Yuri decides to become the demon king and when they return to the capital they hold a coronation ceremony. To complete the ceremony he has to stick his hand in a waterfall but when he does he gets sucked back to Earth.

Yuri awakens to finds himself in the girls' bathroom he originally disappeared from with Ken Murata in front of him with a police officer. Yuri decides to continue his interests in baseball by starting his own baseball team. While Yuri is at a baseball game, his mother, Miko, reminiscences about the past. After the game, Yuri is sucked back to the other world, where he finds out that some humans are threatening to start a war with them. Günter and Gwendal think it's a good idea for Yuri to hunt down the demon sword, Morgif, to make them so strong the humans will give up on wars.

Yuri, Conrad, and Wolfram set sail on a ship to Van da Via island where the demon sword, Morgif, is rumored to be located. On the ship there Yuri meets a ships apprentice named Rick, a merchant named Heathcrife and his daughter Beatrice. That night a group of pirates board the ship they're on. Conrad hid Wolfram and Yuri in a closet from the pirates but Yuri makes a cat noise and gets them caught. Yuri tries reason them that their actions are wrong, but when he sees one of the pirates dragging Beatrice he is angered and brings out his magic. He successfully repels the pirates with a monster made of the bones the humans have thrown on the ground. Conrad, Wolfram, and Yuri are put in a cell but Jozak frees them and they escape in a boat.

Conrad, Wolfram, Yuri, and Jozak reach the Van da Via island. At the inn they are staying at they learn about a legend of a cursed hot spring in the mountain and they decide Morgif is probably there so they decide to go investigate it. They find the sword at the bottom of the hot spring and Yuri attempts to grab it but it has a face and it bites him and so he runs away scared. They give up and return to the inn but later in the night he overhears a conversation between Jozak and Conrad and so Yuri goes back by himself to prove that he is worthy of being king and this time he successfully obtains the sword.

With Morgif now in his possession, Yuri finds out that the sword needs to absorb souls for energy. Jozak tricks Yuri to participating in a gladiator event so he can absorb souls when people die. Yuri is forced into a colosseum fight to the death and when he reaches the field he finds out that his opponent is Rick, the apprentice from the ship they were on.

While Yuuri is on Earth, Stoffel is in the other world hatching a plan. Yuri returns to the other world once again after a baseball game, arriving in the Original King temple's fountain. There he is met by his advisers, and the temple Maiden, Ulrike. Ulrike warns Conrad of an impending danger. At night, Conrad and his soldiers go to investigate some incidents of villagers being attacked. During the investigation, shadowy figures infiltrate the castle.

In the morning, nothing appears to have happened. However one of the soldiers found the door to the treasure room was picked. They find that the gem of the dragon king, that was part of a crown, was stolen. Yuri attempts to find the gem which leads to some comical events. During the search, the shadowy figures reveal themselves and try to kidnap Yuri. Conrad and Wolfram interfere and manage to capture one of the criminals and they find out from the interrogation that Stoffel was behind the stealing of the gem. Later at night Conrad, Wolfram, and Yuri set off to find concrete evidence against Stoffel.

As Yuri, Wolfram, and Conrad leave the castle to some soldiers, Stoffel and Raven start their next move. During the investigation, Conrad orders Wolfram and Yuri to stay put in a building. Yuri drags Wolfram and the remaining soldiers to conduct his own investigation of the missing dragon gem. Yuri's action led to his own abduction by Stoffel's men. Now Conrad and Wolfram head to Stoffel's castle to rescue Yuri.

Yuri wakes to find himself in Stoffel's castle. Stoffel tries to persuade Yuri to join his side, but Yuri rejects. As Gwendal and Stoffel prepare their soldiers for war, Jozak, dressed as a maid, manages to sneak Yuri out of his room by dressing him as a maid as well. Yuri gives their position away when he hears that a civil war is about to start. After Yuri reunites with Conrad and Wolfram, Yuri proceeds to stop Gwendal and Stoffel's forces from engaging. Yuri uses his power to stop the two armies from clashing. After the short battle, Yuri returns home to Earth.

Yuri goes to see a dolphin show, but when he is called up to the stage to shake hands with a dolphin he is then transported to the other world. After Yuri finds out about an imposter thought to be the Demon King, Yuri plans a trip to save the person, accompanied by Wolfram and Conrad. Along the way they met up with Gwendal. During the trip, they encounter a Sand Bear and Wolfram along with the rest of the soldiers except for Conrad and Gwendal fall into its sand trap. Yuri orders Conrad to rescue Wolfram. Gwendal and Yuri continue on to the country of Svelera. When Gwendal and Yuri stop at a town to gather water and provisions, they are mistaken to be an eloping couple and are chained together. They manage to escape.

Gwendal and Yuri enter the capital city of Svelera. When they enter a church they met Nicola, a bride, who then runs away from her own wedding ceremony with Yuri and Gwendal (both still chained together). A young boy gives refuge to Yuri, Nicola, and Gwendal from their pursuers. As they hide from their pursuers, Nicola reveals that she has the demon flute. Yuri tries to use it, but does not work. Their pursuers found their hiding place and so the boy and Nicola escape through the secret tunnel, while Yuri and Gwendal allow themselves to be captured as an eloping couple.

Gwendal and Yuri are sent to trial for Yuri eloping with a demon. Yuri is sentenced to work in the esoteric stone mines, while Gwendal is confined in a cell lined with esoteric stones. Conrad and Wolfram find the boy and Nicola who inform them of Yuri and Gwendal's possible locations. While Yuri was working at the mine, he sees some soldiers attempting to bury a beaten and bruised, but still alive baby. Angered by their actions, he then goes berserk and unleashes a mud monster on the soldiers. Gwendal manages to stop Yuri from causing any more damage.

After Yuri wakes up, he finds Norica digging through the cemetery looking for her son. Instead of a body they find the second part of the demon flute. As Yuri, the remaining women, and the brothers traverse the desert to escape the forces, they are trapped by the Sand Bear and prison guards. Yuri plays the Demon Flute which summons a storm that stops the prison guard army. Ryan then assists their escape with his partner, the sand bear. Back at the castle, Yuri tries to return home, but finds out he cannot go home.

After Yuri studies and practices baseball with Conrad, the news of a demon stone being stolen perks Yuri's interest. Yuri sets off into the city to find the stone with Jozak, Conrad, and Wolfram. They find the missing Demon Stone in the hands of Rosetta and her family, but the family is using the stone's power to repair magic tools and restore magic to earn more money for food. Yuri then decides to help them to get enough food. While they were helping Rosetta, they found that some thieves are trying to reclaim the demon stone. That night they find out that the thieves kidnapped Rosetta's brother and Rosetta set off with the demon stone to exchange for her sibling. Yuri, Conrad, Wolfram, Jozak, and Günter arrive in time to get the demon stone and save Rosetta's brother. Conrad decides to let Rosetta use the Demon Stone for business.

Stoffel and Raven attempt to appease Yuri by arraigning a celebration for him. Celi bursts into the room and declares she is marrying Raven, which shocks Raven and Stoffel. The past of Stoffel, Raven, and Celi are also revealed. During the ceremony, Ryan along with the sand bear, Keiji, arrive. Keiji's entrance however causes a sink hole which Stoffel, Raven, Celi, Yuri, Gwendal, and Conrad fall into. Raven and Celi reminisce their childhood memories at a crystal cave. Stoffel, Yuri, Gwendal, and Conrad find Celi and Raven. Then Keiji, Ryan, Günter, and Wolfram also manage to find them as well. Celi leaves again.

Yuri and Wolfram enter the guesthouse to get rid of some monsters. They both fall to the basement of the building due to weak floors. There they meet some caterpillar creatures that soon transform into their cocoon stage. Wolfram and Yuri have an argument about killing the monsters or letting them live. Meanwhile Gwendal and Günter search for Wolfram and Yuri. The building was collapsing, but then the cocoons hatch into bear bees and they keep the building from collapsing.

A fantasy world calls for a fantasy adventure, and Yuri has decided that he wants to see a dragon. He, Wolfram, Conrad, and Günter set out to the dragon preserve (dragons are an endangered species after all) to find one. Yuri quickly makes friends with one, just in time for some trespassing humans to attack looking to kill and sell a dragon. Unfortunately for Yuri, the group includes a young hero named Alford with a holy sword who decides to use the confusion to try and kill the "evil demon king".

Alford learns what it truly means to be a Hero. Yuri and the others find the dragons a new nest to help shield them from possible future poachers.

A young girl shows up claiming to be Yuri's illegitimate child, and then tries to kill Yuri. Yuri, compassionate as always, decides to give the girl the benefit of the doubt and assures everyone that there must have been a good reason behind it.

Greta spotted a man in town that she wanted to see again. Despite the fact that Conrad had told them to stay put, and still suffering from a twisted ankle, Yuri takes Greta into town.

The masked man turns out to be Hube, who now lays dying with Greta clinging to him. She tells the others that in her old home, Hube, who was in the dungeon, was the only one who would talk or listen to her.

Hube is returned to the Demon Kingdom and it seems that no one likes him. Greta tells Yuri that Hube had once told her that he'd done something unforgivable.

Hube once again attacks Yuri and Conrad defends him. Gwendal decides that Hube must die for his crimes.