Intelligent, young noble William Twining maintains his spot as the top student of his school before the holidays start. When the principal tells him that the tuition hasn't been paid, William goes home to ask his uncle, only to discover that they had gone bankrupt. In order to find a way to pay for tuition, William breaks down the door to the secret family treasury, cutting himself in the process. His blood drips onto a drawing on the ground and a man appears, calling himself Dantalion, Grand Duke of Hell, and tells William to elect him to be king of Hell. Being a realist, William refuses to believe Dantalion's words about demons and has him dragged away by the police. Curious, William goes back to the treasury to find Dantalion's "trick", only to be sucked away into Dantalion's mansion. He meets Baphomet, the goat butler, who explains the election for interim king and he tries to sort things out. Suddenly, the mansion crumbles after another demon, Gilles de Rais, attacks to kill Dantalion. Since Dantalion was still trying to get back to Hell, Gilles decides to take William instead, but is stopped once Dantalion arrives. The two fight ferociously, their power slowly destroying the warp space between the human world and Hell. William, not wanting to be stuck in Hell, orders Dantalion to stop, and Dantalion unwillingly does. Having learned of William's status as the "Elector", Gilles leaves to report back to his master. Dantalion grabs William and calls him Solomon and asks if he has really forgotten him, but William stays silent. Later, William returns to the human world and to school, where he learns that his tuition was paid by an anonymous donator. Wondering whom the mysterious donator is, William sits in class when Dantalion comes in and introduces himself as a new student.
Isaac attempts to summon a demon to form a contract with it, but another candidate for Lucifer's throne, Sytry, disrupts Isaac's ritual, and demands the latter to take him to the Elector or he will lose his soul. Meanwhile, Dantalion reveals to William that he was the one paid for his tuition and that he will stay until he is chosen for the throne. William refuses to be indebted to him and asks a friend and fellow perfect, Mycroft Swallow, to give him scholarship forms so he can pay for his tuition on his own. Dantalion becomes popular with the jocks at school, but later leaves to attend business in Hell. Using Isaac, Sytry approaches William as an underclassman and proposes that his uncle could be his "patron" after William meets him. Desperate to get rid of Sytry, Isaac tries to summon an angel to defeat the former, but summons a demon boar that chases after him and William instead. Sytry kills it and reveals his true nature as the viscount and twelfth noble of Hell and one of Lucifer's candidates to a uncaring William. To force William's hand, Sytry captures Isaac, who is then saved by Dantalion. The two demons engage in a battle until William forces them to stop, who then declares that he will not elect anyone for the throne before leaving. While Isaac takes delight in William's status as the Elector and a descendant of Solomon, Sytry informs William that he will be staying at the school, a fact that he reluctantly accepts. At William's manor, Kevin opens a box containing the ring that once belonged to Solomon that given was to him by God.
Pastor Crosby is alerted of Lucifer's candidates' competition to become the interim ruler of Hell, and sets out to exorcise them. Becoming increasingly popular with their classmates, Dantalion and Sytry continue to enjoy school life at Stradford, much to William's annoyance. William goes to the church and inquires if Crosby he could exorcise demons, although he does not reveal Dantalion's and Sytry's identities to him. Later on, William orders Dantalion to stay away from him, but is then warned by the latter that his orders will only work on Solomon's seventy-two pillars and that more demons will come after him. In addition, Dantalion asks why William cannot remember him in his past life, angering him. At night, William, Isaac, and Sytry heads into the woods to investigate a rumor of a ghost haunting their dormitory. They discovered that it is actually a low-leveled demon under the command of Crosby, who lures them to the church to weaken Sytry's powers in order to kill him. Dantalion arrives and offers to save Sytry if William requests it, who refuses because he does not to rely on Dantalion. Crosby informs Dantalion that, despite William being a human, he will kill him due to his status as the Elector. Dantalion easily defeats Crosby, but is unable to deliver the finishing blow when William orders him to stop. While William decides to no longer deny the existence of demons, Crosby escapes to relay William's status to his organization, the Hand of God. The next day, the headmaster introduces the new priest of the school, who turns out to be William's butler, Kevin Cecil.
William expresses his surprise to Kevin about the latter's position as the new priest at Stradford, who then requests William to keep his distance from him at the school. William is shocked by Kevin's suddenly cold demeanor, but acquiesces to his wish. Later that night, William and the dormitory mother, Maria Mollins, confronts three boys who broke a window. The next day, he overhears the boys planning to break into Maria's room to find something to blackmail her with, much to his irritation. While skipping boating lessons, he spots the Head Boy, Nathan Caxton, acting suspiciously around Maria's room. Shortly after, William barges into her room when he heard a crash and finds a magic circle drawn onto the floor. During the boat race, Nathan exposes his identity as a demon to Dantalion and William when he uses his power to save Maria from a broken oar flying in her direction. At night, Sytry informs William that Maria opened a portal from Hell in order to summon a demon to form a contract with it. The demon attempts to attack Maria, but she is then saved by Nathan in his demon form. Dantalion arrives and explains that Nathan's true name is Camio, a half-demon born from a human woman, one of Solomon's pillars, and a candidate for Lucifer's throne. Maria and Camio reminisce over their past and her desire to become a demon when she dies from her lung disease. Dantalion advises Camio to let Maria die as a human rather than becoming a Nephilim, and they both agree. On Maria's suggestion, Camio decides to remain in the school so he will not be alone once she is gone. Before Maria retires from Stradford to enter treatment for her illness, the three boys that broke into her room comes by and apologizes for stealing an old photo of her and Camio.
Solely focus on his education and tuition, William remains unconcern about the election for the interim ruler of Hell and proceeds with life at school. Camio informs William that an old water mill located at the back of the Headmaster's House is preparing to be torn down to make room for a new building at the school and wants him to catch the students who have been causing a ruckus at night with an odd song before the school's chairman comes to check on the construction for it. William accepts the assignment, thinking that it might help with his tuition if he wins the headmaster's good graces. Dantalion dislikes the fact that William seems to favor Camio and gets into an argument with him about the election and how William still refuses to rely on him. Afterwards, William, Isaac and Sytry goes to investigate the old mill, but left after Isaac was frightened by the rustlings in the forest. Meanwhile, Dantalion visits his guardian, Astaroth, to talk about Camio and she warns Dantalion that he be should more concern about another powerful person close to William. At night, William and Isaac wake up to singing and find a goblin-like creature lurking in the mill. Sytry tells them that the creature is a killmoulis, a fairy that has to mill grains or it will die. William sends the killmoulis to India to work at Isaac's father's tea and spices factory as a solution, but is disappointed that he was unable to receive any merit for his efforts. Later that evening, Dantalion confronts Kevin to inquire him about his true identity and boast that he will uncover all of the latter's secrets.
Still hurt by Kevin's distant behavior, Isaac encourages an upset William to invite Kevin to family day at the school to find a chance to talk to him. During the event, Mycroft asks William to let him take the day off to avoid meeting with his father, Adrian Swallow, due to the pressure of being sent to a military school after his father's near-death accident. Adrian meets William, slightly disturbing the latter when the former invited him to dinner, but Kevin intervenes before William could accept. On his parents' behest, Mycroft requests that William comes to his manor to meet his family. Sensing Beelzebub at work, Dantalion and Sytry tags along to protect William, although they have no intention to interfere with Beelzebub's affairs in accordance to demon law. William dozes off at the party being held the manor and awakens to find that Adrian's corpse is possessed by demon, Eligos, one of Solomon's pillars and current subordinate to Beelzebub. Eligos attempts to kill William for her master, but Dantalion saves him. When Sytry and Mycroft enter the room, Eligos plans to take Mycroft's soul once she realizes that she is outnumbered. William wishes to save Mycroft, but Sytry stops him and explains that Eligos has the right to take Mycroft's soul because of the law. Dantalion volunteers to save Mycroft if William elects him to rule Hell, but then an exorcist and servant of Kevin, Raguel, arrives and tries to exorcise Eligos, who escapes. Returning to school, William and Kevin reconcile their relationship. Later that day, Kevin is revealed to be the archangel Uriel as he speaks to Raguel, who pleads with him to leave the school. However, Kevin is determined to stay so the demons will not take Solomon's soul.
Beelzebub warns Camio things are different now that the latter's identity is revealed, though Camio replies that he is now interested in the covenants he and Dantalion made to Solomon long ago. Meanwhile, Stradford is putting on a production of the play Hamlet with William as the director, Dantalion as Claudius, and Sytry as Ophelia. Later, Gilles de Rais comes to the school to remind Dantalion and Sytry that they must attend the annual Great Sabbat in Hell, which will also be attended by the Four Kings. At the party, it is revealed that Camio is the rumored offspring of Lucifer and has power surpassing that of the Four Kings. Sytry's uncle, Baalberith, sends a horde of demons to the human world to kill William in a plot to take Solomon's soul. After Dantalion flashbacks to a memory with Solomon and remembers that it was Solomon's wish to die by Dantalion's hands someday, Amon informs Dantalion about Baalberith's plan. Sytry, Dantalion, and Baphomet rushes to the human world to protect William, who still refuses to elect anyone for Lucifer's throne. When a demon was about to attack William, the barrier that was placed on him by Kevin earlier appears and surrounds Dantalion, oppressing his powers and causing him to go berserk. Before Dantalion could kill William, Camio arrives to stop Dantalion and orders the demons away. At the end of the Sabbat, Baalberith remarks to Astaroth and Beelzebub that their candidates are as infatuated with Solomon as they were in the past. At the school, William brushes off what happened with Dantalion to help ease the latter's guilt. On the night of the play, William is forced to play as Hamlet and scolds Dantalion and Sytry for ruining it with their antics.
Archangel Michael visits Kevin at the school, demanding to know why he is allowing William to roam freely when he knows that the demons are after his soul. Desiring to observe William, Michael poses as a student at the school named Elliot Eden and boldly challenges William to take first place on an upcoming exam. Initially irritated by him, William meets Michael later in the library and befriends him. However, Dantalion is suspicious of "Elliot", sensing that he has "too much light". During the exam, Michael uses his powers to transfer notes from another student's desk onto William's, causing him to be accused of cheating and sent to the punishment room to await his trial. While there, Isaac and Camio visit him, the latter offering his assistance to help prove William's innocence. Michael comes to see William that night, questioning if he would choose Heaven or Hell, but is chased away by Kevin's arrival. At the trial, Michael transports William and Isaac to another dimension and prepares to perform "ecstasy" on William to turn him into Heaven's pawn. Dantalion appears and engages in a battle with Michael, who tells the former that he knew Dantalion when he was human and taunts him of his past cruelty. Enraged, Dantalion defeats Michael and Kevin interferes before Dantalion could kill him. Once Michael returns to Heaven, William is proven innocent with Camio's help back at the trial. In Hell, Baalberith informs Gilles de Rais that there is a possibility that Lucifer will not awaken from his sleep while Astaroth plans to bring William to Hell to protect him from Heaven.
Michael sends his prized soldier, Saint Jeanne d'Arc, to Hell to act as a distraction so Kevin (Uriel) will be able to perform the ecstasy ceremony on William. While looking for Kevin in his room, William discovers his family ring, which causes him to remember the death of his parents and how Kevin consoled him after their funeral. Once there, Kevin sends William away to talk to Michael about Heaven's plan to lure the demons away from William, and the latter threatens to take the former's last wing if he fails to acquire Solomon's soul. Outside, William sneaks the ring out and runs into Dantalion, Sytry, and Camio. They inform him they must return to Hell to deal with an emergency and warns him to be careful while they are gone, much to his annoyance. While Jeanne begins to engage the demons on the battlefield, Kevin tries to perform the ceremony and finds out that William rejects it for some reason. Appointed to guard William, Gilles de Rais goes to stop Kevin, but is deterred by Raguel. Gilles expresses his utter hatred and disgust of Heaven and its angels, as they were the ones that turned his "beloved Jeanne" into their pawn. Raguel tells Gilles that Jeanne is in Hell, but Gilles disregards the information and professes his desire to defile Jeanne to free her from Heaven's ecstasy. To save Raguel, Kevin agrees to cease the ceremony on William. However, Gilles moves in to kill both of them until Dantalion interferes. While Dantalion orders Amon and Mamon to take Gilles back to Hell, Kevin reports to Michael about his failure. Although prepared to take Uriel's last wing, Michael demands that Uriel leaves, distressed that he is beginning to fade from existence.
Stradford is holding their annual culture festival on campus, where the students set up booths in order to compete for the most brooches as part of their curriculum. After seeing how well Sytry's booth is doing, Dantalion and Isaac begin their own booth, both of them becoming increasingly popular. However, William, wanting to use this as an opportunity to raise money for his tuition and boost his grades, is unable to formulate an idea for the festival. Manipulating Dantalion and the others (including Gilles de Rais, who was at the school to help with Sytry's booth), William decides to use to them as workers for his café. Soon, he is approached by Baphomet and Sytry's butler, Leonard, in their human forms and is asked to be a judge for the butlers' cooking competition. Ultimately, Baphomet is the winner, much to Leonard's dismay. Afterwards, William makes Baphomet and Leonard serve as chefs for the café. In the end, while William's café did well, Camio was proclaimed the winner of the culture festival.
Reflecting on his previous plan's failure, Michael meets with Uriel to discuss their next strategy to obtain Solomon's soul. In Hell, the Four Kings unanimously agree to bring William to Hell to protect him from Heaven, and tasks Astaroth to bring William to them. Sent by her mother Astaroth, Lamia appears at Stradford and drags William into Hell through a portal and delivers him to a meeting composed of Sytry, Camio, the Four Kings, and other aristocratic demons. The demons explain the situation to William, who disregards it as the war between Heaven and Hell means nothing to him. Due to William's dismissive attitude on the election for interim king and Hell in general, a demon tries to attack William out of rage, but Dantalion appears and defends him. Solomon's Ring of Wisdom, which William still has in his possession, falls out of his clothes and the demons recognize it as the ring that Solomon used to enslave them. Determined to put an end to Solomon's control over the demons and gain power, Baalberith and his minions moves in to kill William, who slips the ring onto his finger, causing Solomon's soul to take over his body. While Michael and Kevin sense this from Heaven, Solomon easily fends off the demons, including Gilles de Rais (who wanted to kill him to raise his status), and uses his power blessed by God to nearly eradicate Hell. As Solomon greets each of his former demon friends and pillars, Dantalion, realizing that Solomon's tremendous power is intimidating the demons and would destroy Hell, goes to remove the ring to no avail. Shockingly, Dantalion is the only one Solomon does not recognize and the latter inquires on the former's identity.
Solomon, who has forgotten Dantalion's identity, demands that Dantalion should not interfere with his battle against Baalberith and the other demons. Shocked and unable to go against him, Dantalion is beaten down by Solomon's powers while remembering his past history with the latter: Despite being imprisoned by his father, King David, and being labeled as "a child of sin", Solomon summoned demons and subjected them under his control while also befriending many of them. Despite being powerful, Solomon did not raise arms against his father and lived most of his life peacefully with his demon companions, much to Dantalion's bewilderment. After crushing his father's army that was sent to kill him, Solomon took over the kingdom of Israel. Although Solomon showed his father mercy, David refused to acknowledge him as his son and cursed him, much to Solomon's sadness. Soon after, Solomon requested to die by Dantalion's hands. Regaining consciousness, Dantalion saves Baalberith from being destroyed by Solomon and urges William to remember who he is to break Solomon's control over his body. Lucifer awakens from his sleep (Michael and Kevin could sense from Heaven) and meets with Solomon, who then allows Dantalion to remove his ring to release his control over William. With Lucifer awake, the election for the interim ruler is halted and William is returned to the human world. However, William is distraught to learn that no one at the school, except for him (and Kevin), remembers Dantalion, Sytry, and Camio. On his way out from visiting Kevin at the church, Solomon's ring falls out of his clothes and is picked by Dantalion, who smiles and asks William to take good care of it.