Beelzebub

Tatsumi Oga, a delinquent student at the worst high school in Japan, arrives at school with a small baby who emits massive shocks of electricity whenever it cries. Oga claims he found after splitting open a strange man who came floating along the river, though his friend Takayuki Furuichi doesn't quite believe. Whilst at Takayuki's house, they are both approached by a demon woman named Hildegarde who claims the baby, known as Kaiser de Emperana Beelzebub IV, is the son of Devil King who was sent to Earth to find a human to raise him so he could destroy humanity.

Hildegarde presents herself to Oga's family in the wrong way, giving them a different view of the situation. She also informs Oga that if he is more than 15 meters away from Beelzebub, his cries may destroy the city. Whilst dealing with this situation, Oga comes up with the idea that, if he can find someone more strong and ruthless than him, he can dump Beelzebub on him and be free.

When Oga loses some food for Beelzebub and he starts to get hungry, Oga instead uses up a vending machine's worth of yoghurt drink, revealed to be the favourite of the second year delinquent, Hajime Kanzaki. Figuring he may be worthy of dumping Beelzebub on, Oga goes to see him under the pretense of becoming his henchman. After beating his current henchman, Oga is asked by Hajime to throw him out of the window, but instead decides he isn't worthy based on Beelzebub's expression and knocks him out instead.

Beelzebub is currently in a phase of non-stop urination that threatens to flood the city. Hildegarde manages to temporarily stop it with an interdimensional diaper, but in order to stop it completely, Oga decides he needs to get a super absorbent daiper advertised on telly. Upon reaching the supermarket only to find the brand had already sold out, it is taking hostage by a group of thugs. As Oga probes everyone to see how ruthless they are, Beelzebub's diaper gets stuck on the leader's gun. Whilst the leader considers raising Beelzebub, Oga ends up changing his mind and beats him, before they decide to hover Beelzebub over the ocean to finish urinating.

After a sleep deprived night, Oga notices he has a strange mark on his hand, revealed to be Zebel Spell, a symbol of the contract between him and Beelzebub. Hildegarde and Alaindelon explain that a Demon Lord's parent is a vessel for his power, and the mark will grow the more opponents Oga defeats. Wanting to get rid of the mark, Oga decides to give up fighting in the hopes it'll make the mark smaller. Meanwhile, another delinquent, Tatsuya Himekawa, hires some thugs to kidnap Hildegarde and Takayuki in order to get Oga's attention. Oga uses Alaindelon to sneak in and forgoes his promise to beat Tatsuya's henchmen. Tatsuya goads Oga into hitting a cermanic plate hidden under his shirt, but his punch is so powerful it knocks him out anyway. Tatsuya then tries using a stun rod, but Oga resists it and uses Beelzebub's power to beat him, which in turn makes his mark larger.

The mark on Oga's arm goes back to its small size, much to the chagrin of Hildegarde who destroys his games console. She brings a play room for Beelzebub to play in, but upon building it, it turns out to be a magical cage that both she and Oga get stuck inside. The only way to get out is to have Beelzebub solve a block puzzle toy, but he just ends up breaking it. Frustrated, Oga utilises his Zebel power to punch his way out, though it makes his mark grow big again. Rummaging through a bag of demon world toys, Beelzebub opens a chest that transforms Oga and Takayuki into nurses, which can only be reverted by helping Beelzebub play Doctor. Oga decides to use the opportunity to try and do good deeds and get rid of his mark, though has trouble handling the demonic medical supplies. After some failed attempts at treating some of the students, they change back after discovering an actually useful tool on Beelzebub's costume, but accidentally triggers an explosive that negates all the work Oga did getting his mark down to size.

Oga's family coerce him into taking Beelzebub to the park so he can make his 'park debut'. Despite making a bad impression with the other mothers, Oga meets a girl named Aoi who is looking after her little brother, Kouta. Aoi tries to help Oga make his park debut which, partly due to the appearance of Alaindelon, doesn't reap much success. The general antics convince the other mothers to call over a police officer, who starts badmouthing both Oga and Aoi. Before Aoi can lose her temper, Oga throws him into a trash can, with Aoi coming off impressed. After Oga leaves, it is revealed Aoi's true identity is that of Aoi Kunieda, leader of the Red Tails gang and one of the Tōhōshinki.

Aoi and the Red Tails return to Ishiyama High to deal with Oga, despite not knowing who he is. When Takayuki drags Oga to meet her, Aoi is shocked to find Oga is the one she met at the park, although he doesn't recognize her back. Aoi starts to fight him using powerful bokuto attacks that cut through glass, but he manages to dodge all of her attacks. Impressed by her strength, Oga asks Aoi to become Beelzebub's mother which causes her to become embarrassed and run off. Afterward, Aoi seems to develop feelings towards Oga. Meanwhile, another delinquent gang plots to take down Aoi.

The delinquent group, MK-5, come to confront Oga, but are promptly beaten by him and Hildegarde, who catches the attention of a smitten Aoi. As Aoi's right hand ladies, Chiaki Tanimura and Nene Oomura confront Oga, Hildegarde, who had heard from Oga about his plans to dump Beelzebub onto Aoi, challenges her to a fight to test her strength. When Aoi returns to find Chiaki and Nene injured, she assumes Oga is responsible and confronts him in a fight. In actuality, Chiaki and Nene were attacked by MK-5, under the orders of Miwa, who desires the title of Queen. Shintarou helps Nene get to Aoi to tell her the truth, but when Miwa makes his move, Oga promptly beats him up. Afterward, Aoi is once again baffled by Oga trying to give her Beelzebub, explaining that he's forced to raise him because of Hildegarde. Later, Aoi decides to quit the Red Tails and put Nene in charge, feeling that she is no longer able to serve as their leader.

A stray cat becomes attached to Oga, causing Beelzebub to become extremely jealous and shock anyone he shows affection towards. Beelzebub challenges the cat to a fight and subsequently loses, but when a gang of fierce cats gang up on the cat for fighting in their territory, Beelzebub stands up for him, prompting Oga to help out. Afterward, Beelzebub and the cat become friends. Later, Oga takes Beelzebub and the cat to a play center where he once again meets Aoi in her park getup. While Beelzebub and Kota getting into heated competition with some other toddlers, Oga and Aoi have a conversation full of misunderstandings, both ending with explosive results. Afterward, the cat decides to leave in order to protect his fellow strays.