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.

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.

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.

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 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.

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.

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.

The students of Ishiyama High are ordered to do physical examinations amongst themselves. Baby Beel gets jealous of a toy which accidentally gets stuck to Oga's back, and locks himself inside of a locker to pout in protest. While trying to lure him out, the girls come into the room for their physicals, forcing Oga and Furuichi to hide under the desk while the girls strip off and start measuring. The girls leave the room without seeing them, despite a few close calls, and upon the girls' departure Oga and Furuichi get up from under the table and start trying to lure Baby Beel out once more. Aoi returns to the classroom moments after leaving, and finds the two of them in the room. She realizes that they have been there all along, and goes on the offensive. During the conflict Baby Beel exits the classroom through an open window and climbs into Sadohara's sports car, and locks the door behind him, which Oga is unable to pull open. Baby Beel "springs a leak", filling the car. Oga, unable to think of any other option, chops the car resulting in the entire shell of the car shattering.

Oga becomes tired of practicing and walks out of the gym, itching for a fight. He finds Miki training in a nearby Dojo, and challenges him to a fight. Oga takes all of Miki's attacks head on, and is clearly damaged from the strength of the attacks. Miki claims his technique is only at 70% power, but will be ready by the time the school festival arrives. Miki has a flashback to a time when Oga betrayed him in the past, where Miki followed Oga to fight alongside him, but Oga turned on him, knocking him down and kicking him on the floor. Oga finds Tojo and challenges him to a fight, looking for some insight into what his own special technique should be. Oga appears to be quite beaten up after fighting Tojo, but not deterred from it at all. He claims that it is all a matter of fighting spirit, and that he is ready to win at volleyball and any fights he gets into.

Kiriya, the man who scarred Miki and tried to attack Oga in the past shows up at the volleyball match. He declares that he is seeking revenge on Oga, and has taken hostages at the school to stop anyone interfering. Kiriya reveals that he had to move away to Nara previously, and this is why he is only now seeking revenge on Oga. Miki, who also moved to Nara after being beaten by Oga, realizes that Oga had only beaten him to protect him from Kiriya when the two of them were both in Nara. Oga and Miki team up to take down Kiriya's henchmen while the 6 Holy Knights, the Red tails and the Tōhōshinki incapacitated all of the Keimo High students holding the St. Ishiyama students captive.

Baby Be'el is having a great time at a railroad crossing and wishes Oga to sing with him. Oga, however, is thinking about the Izuma's words about demon powers. Aoi, who is happens to be behind them, runs into Nene and Yuka, and they once again assume Aoi is going to confess to Oga. At school, Oga has become a celebrity for the "magic trick" after the volleyball match. Furichi drags Oga into creating another illusion a.k.a. making Furichi disappear by sending him flying using his mindless violence. Aoi, not convinced, confronts Izuma; however, she doesn't make any progress. Later, Tōjō talks to Shizuka stating his fight with Izuma isn't over. The two meet on the roof, and Oga joins in. However, before they can begin, Saotome beats them up and brings the unconscious Oga and Toujou to clsss.

Whilst everything is in commotion, Lord En continues his tour of the human world and is now in Akihabara. As usual, he also cries due the maids being mistaken for cosplayers and he feels left out. Also, Furuichi is called a pervert by Alaindelon's daughter. Kanzaki and Yuka are both spying on Tojo and Izuma's fight, until two of the pillars: Graffel and Naga interrupt the fight and beat Izuma up. Izuma is revealed to be only half demon and hasn't actually made contact with any real demons before and so his power is weaker than that of a real one. Tojo then cuts in and so does Kanzaki however Graffel easily defeats Kanzaki, and Tojo fights till he is unconscious, however they soon leave to join Hecatos. Oga attempts to beat Hecatos using his Zebel Blast, however is blocked easily. Saotome then enters and uses his spell master magic forcing Graffel, Naga and Hecatos to retreat. Quickly, they call Lamia and Dr. Bloo to check on Hilda before it's too late.

Oga begins his training with Ittousai while Furuichi and Lamia enlists the help of their Ishiyama classmates to search for Lord En. Can Oga and Beel complete their training in time and will Furuichi and Lamia find Lord En before he destroys the city with his army?

Due to the after effects of Super Milk Time, Baby Beel and Oga have swapped bodies. In order to return things back to normal, they try to look for Saotome-Sensei to see if he knows how to, however they are continuously disrupted by everyone else from the academy trying to start a fight with Oga (who is currently Baby Beel). Eventually, Baby Beel runs away after being drawn to a smell, and Hilda, Furuichi and Oga try to catch him, where they find him attached to Aoi. After hugging Aoi, BabyBeel runs off once again and so Hilda and Aoi have a match to see who can catch Baby Beel. Although at the end BabyBeel chooses Aoi over Hilda after she shows some of her new demon magic from training. Not long after, Aoi and Hilda attack Oga(Baby Beel) when fighting over him, and just before they hit him Oga and BabyBeel return to normal, leaving Oga being severely hurt by the two and BabyBeel perfectly fine. Saotome-sensei then comes to school the day after.

Oga ,Furuichi and their teacher (which was the Spellmaster) was searching for the black version of Baby Beel. Oga thought that it was so bored that it ran away. But actually it was because of Himekawa.They asked some girls if they saw someone with a baby, and they answered that they saw a nice looking, hot guy that smelled so nice. He took it because he believes that it was the key for their family to be more successful. In the end, Black Baby Beel was reunited with Oga's teacher.

Behemoth visits St. Ishiyama to find Oga and Beelzebub, but he gets welcomed by Zenjuro, Ittousai, and the principal. Hilda holds Oga down while they watch the fight. Meanwhile, Furuichi and the delinquents arrive at Akumano Academy. They stumble upon the Pillar Generals. Agiel attacks only to meet Aoi who arrived in the nick of time.

Will Aoi defeat Agiel?