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.

Himekawa plots revenge against Oga by setting him up with a poisoned sandwich, but it doesn't affect him due to his resilience to Hilda's cooking. He next attempts to use a popular costumed character in order to try and capture Beelzebub, but Oga and Beelzebub notice that he's a fake due to differently colored chopsticks. He later attempts to seduce Takayuki into giving him Beelzebub, but Alaindelon intervenes, transporting Himekawa and his gang to a far off island. The resulting failures leave Himekawa with no money to hire any more henchmen. Ans as he tries to attack Oga by himself, he gets caught up in fighting against a rival gang also attempting to kill Oga. Afterward, Kanzaki offers him one of his yogurt drinks, to which Himekawa repays him with a year's supply with a month long lifespan.

Beelzebub has been crying all night for the past three nights, which is causing Oga and the whole neighborhood to get very little sleep. Wanting to stop his night crying to avoid being chased out of their house, the Oga family attempt to play with Beelzebub all day in order to tire him out come nighttime. Despite managing to get him to sleep, Beelzebub still wakes up and cries in the middle of the night. After Oga spends the day dealing with a sleep deprived Tōhōshinki, Hilda brings a toy that calms Beelzebub down, but is even noisier than his crying.

As Ishiyama High holds an assessment test for its studyphobic students, the Great Demon Lord also holds a test for Beelzebub, requiring him to earn points by beating up various things, or run the risk of both he and Oga facing remedial classes in the Demon World. With Beelzebub not showing much hope, Oga decides to try and dump Beelzebub on whoever scores highest on the test. When a rival school interrupts the test, Oga fights them off to allow the other students to keep on the test, earning enough points to avoid remedial lessons, later learning that the smartest student it actually Takayuki. Later, Hilda prepares a letter to the Great Demon Lord, writing reports on Oga, his family and the Tōhōshinki.

As Kanzaki tries to come up with a new special move to beat Oga, his followers suggest the key to Oga's power is having Beelzebub on his back. Thus, Kanzaki starts training with a weighted baby doll on his back. When that plan doesn't work, they try finding a living substitute, which end in more embarrassing results. Meanwhile, Oga takes Beelzebub to the park where they once again meet Aoi, who has lost track of Kouta. Kanzaki coincidentally finds him and deems him a worthy substitute for Beelzebub. Whilst playing with him, he figures out the key to performing his new special move. After returning Kouta to Aoi, Kanzaki performs his special move, though it just leaves everyone confused. As Kanzaki leaves, Oga admits his move was surprising.

Takayuki tells Oga about the final member of the Tōhōshinki, Hidetora Tōjō, mainly to get him interested about ditching Beelzebub so Alaidelon will leave him alone. Wanting Oga to get beaten, Himekawa informs them that Tōjō works at the beach. Oga and Tatsumi go there the next day and get mistaken as part-timers for a yakisoba stand, later finding that Tōjō had actually been fire the previous night. As he keeps missing him, Oga asks Aoi to pass on a letter of challenge to Tōjō, whose henchmen find it before Aoi can rip it up. Oga gets held up to their meeting, arriving only finding a pile of beat up gang members, which Tōjō beat all by himself. Despite Aoi's warnings, Oga looks forward to finally meeting Tōjō.

Takayuki is dragged away from his tropical summer vacation against his will to join Oga and Beelzebub to a public pool, though Oga is soon shocked by the fact he could've gone to the tropical resort himself. Whilst chatting with the Red Tails, Takayuki is approached by some delinquents from his old middle school and ends up going against their leader, Takashima, when they start hitting on the girls. Takayuki tries to hide in the pool, but Takashima calls more of his men and tricks the lifeguard into evacuating the pool, leaving Takayuki and a stunned Oga. As Takayuki becomes surrounded, he forces Beelzebub to cry in order to shock all the delinquents before facing Oga's wrath.

The Demon King sends Beelzebub some summer homework, containing a notebook which brings anything drawn in it to life. Next, Hilda brings Oga to Ishiyama High to collect prints of rare, powerful beasts, aka the delinquents, by whacking them with a giant fly swatter. During the chase, Beelzebub ends up planting several carnivorous Demon Plants, one of which grows into a massive giant causing havoc until Oga destroys it.

Beelzebub gets a summer cold, causing his body to give off incredible heat. As his condition gets worse, the Zebel Spell on Oga's hand disappears, leading Hilda to become frustrated towards Oga for showing little care towards him. As Oga tries to decide what to do, he encounters one of Tojo's henchmen, Shojo Aizawa, who seemingly outmatches Oga before leaving. The next morning, both Hilda and Beelzebub have disappeared. As Oga searches for them, he encounters Tojo with Beelzebub on his back, who challenges Oga to a fight. As the two fight, Oga is shocked to see what appears to be the Zebel Spell on Tojo's shoulder, before he is knocked clean into the river.

Oga returns home after being defeated by Tojo to find that Hilda has returned from the Demon World with the imperial doctor, Forcas Rachmaninoff, and his assistant, Lamia, to help determine the cause of Beelzebub's illness. He diagnoses the illness as King Fever and when Beelzebub was trying to draw out massive amounts of demonic power, he accidentally severed the link between him and Oga and found Tojo similar to Oga and connects to him. Oga begins a treatment to try and reconnect with Beelzebub and enters a dream world through the effects of a Demon World drug that is shot into his head where he eventually finds a sleeping Beelzebub in the middle of several inscribed circles that are shrouded in an electrical field. After about 6 hours of treatment, Oga still has not waken up and Lamia assumes that he has died and becomes happy. Oga suddenly wakes up and spanks Lamia and claims he is teaching her some manners. Hilda asks Oga how he feels after having gone through the hard part of reconnecting with Beelzebub and Oga says he is in the mood to get Beelzebub back.

Oga reflects on Lamia telling him to take a shower before going to Tojo and sees a silhouette outside his bathroom window and thinks it's Lamia, but it's really Shiroyama, who takes Oga to Kanzaki and Himekawa who tell him they want to team up to beat Tojo and Oga agrees. On the way to Ishiyama High, they are surrounded by a large group of delinquents, which Oga takes care of swiftly. Upon arriving at Ishiyama High, the rest of the school is there to confront Oga, only to have Kunieda and the Red Tails take care of them for him. Tojo sets off fireworks in order to try and cheer up Beelzebub and all the delinquents are captivated by them. Oga confronts Tojo and Tojo is willing to give Beelzebub back only to have Oga tell him that whoever Beelzebub wants to stay with is up to him. Beelzebub's fever breaks and he immediately goes to Oga and climbs on his head, and a flashback shows Oga reconnecting the bond between him and Beelzebub. Just as the episode ends, Oga and Tojo are about to begin their fight.

Furuichi is woken up by Alaindelon at 4:00 A.M. and is teleported to Ishiyama High to watch Oga and Tojo's fight with Hilda, Lamia, and Forcas Rachmaninoff. Meanwhile, Kanzaki, Himekawa, and Shiroyama confront Aizawa and Karou and begin to fight, but are quickly beaten by them, but they refuse to give up because of their stubborness. Natsume shows up and defeats Karou in one blow, and is about to deal with Aizawa. Kunieda and the Red Tails finish dealing with the delinquents and go to watch Oga and Tojo's fight, with Oga at a disadvantage. Beelzebub activates the Zebul Spell, thinking Oga cannot win, shocking everyone. Lamia accidentally sets off one remaining firework and it's aimed at Oga and Tojo. Oga easily stops the firework and causes a giant flaming Zebul Spell to appear on the ground. Oga orders Beelzebub to not interfere in his fight, promising he will win and Beelzebub erases the Zebul Spell. Oga and Tojo resume their fight, which goes on until dawn, and just as it looks like Tojo is about to win, Oga spins around and performs a suplex on Tojo, winning the fight. Oga tells Beelzebub he won the fight and Beelzebub tackles Oga to the ground. Tojo's past is revealed, showing a man he looked up to who taught Tojo the ture meaning of fighting and had a Zebul Spell on his shoulder, which Tojo got tattooed on his shoulder in order to look like him, and Tojo tells Oga he is now the strongest at Ishiyama High.

Baby Beel's fear of insects prompts Oga to give him some special training up in the mountainous village of Mapputatsu. There, he stumbles across Kunieda carrying Kōta in the disguise she wore at the park with her grandfather. While Oga is still unaware of her true identity as Kunieda, her grandfather develops an interest in him.

As the Zebub Spell is restored, Lamia and Dr. Forcas begins to return to the Demon World through Alaindelon. While they were being transported, Baby Beel grabbed onto Lamia and refused to let go, causing Furuichi and Oga to be transported as well. All 4 of them landed in Vlad's Haunt, one of the most dangerous areas in the Demon World, with Alaindelon unconscious. Lamia manages to contact Hilda through her cellphone, and is informed that Alaindelon's daughter might be able to transport them home safely. A Yople alien shows up, which Oga punches, causing the Yople to call the creatures in Vlad's Haunt to attack the group. As the group escapes to find Alaindelon's daughter, they encounter numerous creatures, with Furuichi suffering the worst of their attacks. When the group finally reaches their destination, they realise the place was ransacked by bandits, Gare and Edda, who have kidnapped Alaindelon's daughter. As Oga defeats the bandits easily, the episode ends with Furuichi realising how attractive the daughter is.

Finding Alaindelon's daughter rather attractive, Furuichi vows to rescue her while Gare and Edda leaded him to Oga and Lamia in the village of thieves where Alaindelon's daughter, Angelica is trapped. The rescue runs smoothly until a large Demon named Vlad's Master is awakened and terrorizes the village in which Beelzebub suddenly growed bigger and larger to fight it off.

The students of Ishiyama High settle in at St. Ishiyama, with all the significant characters being placed in the same classroom. Their new homeroom teacher, Sadohara, challenges Tojo to an arm wrestle to try to establish some authority over the class, failing miserably when Tojo accidentally slams him upside-down against the wall. He then picks a second target, Oga, who was sleeping in his seat, and wakes Oga in order to challenge him. Baby Beel is angry to have been woken and subsequently tantrums, shocking Oga and Sudohara.