Beelzebub

Oga Tatsumi is a strong, ruthless high school delinquent. One day, he rescues a big guy who comes floating down the river. When he does, the big guy splits in half and out pops a bouncing baby boy, Kaiser de Emperana Beelzebub IV (Baby Beel), the son of the Great Demon Lord, who came from the Demon World in order to destroy humanity.

Oga has been chosen to be the parent of Beelzebub. However, every time Baby Beel cries since he lets out a severe electric shock. According to Beelzebub's demon maidservant Hildegarde (Hilda), if he moves farther than 15 meters away from his human parent Oga, the electric shock will reach lethal levels. Oga shuffles to school with Baby Beel, and begins to search for a substitute.

Oga hears about how horrible and ruthless a guy Kanzaki is, and runs into the third year classroom to try and dump Baby Beel off on him. When he sees Kanzaki's evil looking face, Oga thinks that this just might be his chance.

When Oga wakes up one morning, he's floating in Baby Beel's pee, as it is "urination season" now in the Demon World. Oga is concerned about the entire city being inundated, so he takes Baby Beel to the drug store to buy disposable diapers. While he's there, a bunch of bank robbers take Oga, Baby Beel, Hilda and Furuichi as hostages.

On the back of Oga's fist a mark appears that is proof of his contract with Beelzebub, the Zebul Spell. After hearing that the demonic power grows the more he gets into fights and runs wild, Oga decides not to fight anymore. However, Himekawa Tatsuya, heir to the Himekawa Zaibatsu and one of Ishiyama's TKKH like Kanzaki, is secretly plotting to do Oga in himself.

Hilda brought back a bunch of toys from the Demon World for Baby Beel. As Oga and Hilda assemble one, a horrible accident occurs. The Zebul Spell expands when one does evil. So won't it contract if you act like an angel?

When he takes Baby Beel to a park to meet a community of mothers, Oga meets fellow high schooler Kunieda Aoi and her much younger brother Kota.

The Crimson Queen of the Ishiyama TKKH, who holds the deep confidence of all the women, has returned. Hearing the rumors, Furuichi drags Oga along in trying to get a glimpse of her. Before long the Queen appears! She tries to bring the iron hammer down on Oga, who in her absence has defeated two members of Ichiyama's TKKH.

Hilda hears about Kunieda Aoi from Oga and wants to find out more. She goes in search of her to test her skills. At the same time, there's someone with a half-baked scheme against Oga using Aoi's protective attitude.

A cat follows Oga home one day. Baby Beel challenges the cat for Oga's attention. Once the showdown is over, they are surrounded by stray cats, and Baby Beel stands tall to defend. This is the start of a friendship between 1 baby and 1 kitty.

Himekawa lives by the motto, "there's nothing that money can't control." In order to get revenge on Oga, he uses the power of money to work behind the scenes in plan after plan to defeat him, but Oga keeps getting the upper hand.

Oga's family is seriously worried about Baby Beel's night crying. They try all kinds of things but nothing works. Oga heads to school, completely exhausted, as do the other students looking for a fight.

It's the Ability Assessment tests for Ishiyama High, the den of delinquents. Baby Beel and Hilda have gotten used to various customs in the Human World and report on Baby Beel's growth to the Great Demon Lord.

Kanzaki is thinking hard about why he's unable to be a match for Oga, and tries to perfect a new special move, when Shiroyama realizes the crucial difference: Oga has a baby on his back. Shiroyama and Natsume help Kanzaki go through intense "BoB" training.

Oga hears about the legendary Tojo.

Furuichi is called back from a resort on a tropical island where he was vacationing with his family to go to the public pool with Oga. At the pool, Furuichi runs into a nasty upperclassmen he knew from middle school, and it's not just him, but Nene and the other Red Tails that are in trouble.

The Great Demon Lord decides to send Baby Beel some summer homework on a whim. The gang helps Baby Beel do the homework as best they can, but if they mess up how to use the items in the kit, all of humanity is doomed.

Baby Beel comes down with a fever, and Oga's link to him and his never-ending shocking comes to an end. Hilda and Baby Beel disappear, and when Oga goes looking for them he finds Baby Beel stuck on Tojo's back.

With a single blow from Tojo, Oga learns the difference in their strength and dejectedly returns home. Waiting for him are a former court physician and his assistant from the Demon World. Oga is thrown into his own mysterious mental subconscious in order to grow as the parent of a Demon Lord.

Oga is all raring to go finally beat Tojo this time, and Kanzaki and Himekawa try and join forces with him. Then, delinquents come to attack the three! Making it through the forces, Oga finally arrives for his showdown with Tojo.

The battle to decide Ishiyama's strongest has started. Oga has a tough time against Tojo's beastly strength, but while getting the snot beat out of him he puts his entire body into a single punch that sends Tojo flying back.

Baby Beel and Oga head deep into the mountains to help train Beel to get over his fear of bugs, where they run into Aoi and Kota. Aoi is nervous because it seems like a date, until her master martial artist grandpa shows up, takes the initiative and attacks Oga.

Their work done, Doctor Furcas and Lamia head back to the Demon World, but Oga, Beel, and Furuichi accidentally hitch along for the ride. With Alaindelon out of commission after exceeding his maximum interdimensional transfer weight limit, the gang has no choice but to search for his daughter.

In search of Alaindelon's daughter so they can get back home, Oga and the gang head to a city of criminal Demon Beast poachers to rescue her. But before they get the chance, a giant monster appears to take revenge on the poachers for the harm they've done to Vlad's Haunt.

Oga and Furuichi head off to school to start the new term, except Ishiyama High is still completely destroyed. So instead, all the students from Ishiyama are split up and sent off to different high schools temporarily.