Demon World Arc

The Demon World arc is the second story arc in the series. The manga takes place during Chapters 41-47 while the anime follows Episodes 23-24. The story revolves around how Oga, Furuichi, and Be'el are inadvertently sent to the Demon World along with Furcas Rachmaninoff and Lamia.

Synopsis
Spoiler warning: Plot and/or ending details follow.

Entering the Demon World
Furcas and Lamia decide to go back to the Demon World because Be'el is now better. There is initially problems in communicating with Alaindelon, whom Furuichi notes hasn't been at his house for some time, though Alaindelon is revealed to have spent much of his time talking to his family. Furcas and Lamia decide to return home from the river. Oga and Furuichi accompany the two to see them off, as well as Hilda. As Furcas and Lamia are about to leave, Be'el suddenly grabs on to Lamia's leg and refuses to let go of it. Oga realizes that if Be'el enters the Demon World, which is more than 15 meters away from him, he'll die. Oga tries pulling Be'el away and Furuichi helps after noticing that Oga and Be'el were being sucked in by Alaindelon. However, the two teenagers (and Baby Be'el) end up entering the Demon World along with Lamia.

Vlad's Haunt
Oga and Furuichi are shocked that they're in the Demon World and are approached by one of the more peaceful inhabitants, a Yople Alien. Oga and Furuichi mistakenly believe that the Yople is being hostile so Oga punches him, horrifying Lamia with what they did. She is worried about what will happen next but is already more concerned about where the four are currently at: Vlad's Haunt. She starts telling Oga and Furuichi about how dangerous the situation is but they state that it's not a big of a deal, as Alaindelon can transport them back. Lamia realizes this and feels relieved, but then she and the two teenagers notice Alaindelon's body being pierced by several tree branches. Alaindelon is set on the ground where Oga demands that Lamia heal him, to which she replies that she can't deal with wounds like his. Alaindelon states that he'll heal after he gets some rest and before shutting down, mentions how he'd promised to buy his daughter a dress for her birthday.

The three start to panic but Alaindelon's cell phone starts ringing. Lamia then picks it up and finds out that it's Hilda. Hilda tells the three that she already knows of the situation from speaking to Furcas. She then states how she hopes that they didn't harm any natives but is shocked when she learns that they did. Hilda is even more shocked when she discovers that they harmed a Yople Alien and tells the three to run away. The Yople that Oga punched awakens and summons a wild Ak-Baba, a giant one, to chase after the three. Baby Be'el manages to save the three by electrocuting the Ak-Baba with his own powers. Hilda notices this and then tells the three to find Alaindelon's daughter who can help the three escape Vlad's Haunt. They then start searching for Alaindelon's daughter and eventually find her house on the outskirts, Lamia stating further that she's doing research on the animals in the area. They enter her house, as the door was open, but discover her house being destroyed. Lamia notices that the mess was newly-made and find a stranger in the house, who then disappears via a trandsimensional stone when two other Demons enter the house. Oga quickly disposes of the two Demons, both of whom are later told by Hilda to have been thieves that hunt around Vlad's Haunt, but one of them notices the Crest of the Royal Family on Oga's hand before being beaten. The two thieves are then tied up.

Afterwards, Furuichi notices a picture of Alaindelon and his daughter Angelica. Noticing how beautiful Angelica is, he decides to help rescue her willingly. The three then find a thief's hideout near Angelica's home. There, Furuichi uses the transmitter to ask Hilda for Angelica's name (as he still doesn't know it yet) but before Hilda can answer, the transmitter is cut off. The two thieves from before, having freed themselves (somewhat), tell the group that communication can get blocked once a person is inside a thief's hideout.

The Damsel in Distress
Inside the hideout, Angelica is handcuffed and locked in a cell by the unnamed Boss because they're after the mystical beasts in Vlad's Haunt. Angelica refuses to tell them anything. The Boss then receives word from the two thieves Galle and Edda about Oga's arrival, but only that Oga has an affiliation with the Royal Family which causes him to worry. The Boss orders that security is to be increased and leaves. As he leaves, Angelica pouts about how awful of a birthday it is. She thinks about what Alaindelon told her and says aloud that he's likely to have forgotten it anyhow.

Much to her surprise, she sees Furuichi standing outside by the barred windows (receiving standing support from Oga and Lamia). Furuichi tells Angelica that Alaindelon was going to buy her a dress for her birthday. Angelica then gets up and asks if he is Furuichi, as Alaindelon always spoke to her about him. Furuichi states that he is but Angelica then tells him to run away, as a big Demon is about to come. However, Furuichi's head is stuck between the bars.

Vlad's Master
Suddenly, a beastly Demon arrives. Angelica states that the Demon is Vlad's Master and that it is the guardian of Vlad's Haunt. The guardian has come because the thieves have been capturing so many of the other beasts in Vlad's Haunt. She tries to help Furuichi escape but only ends up injuring him. Afterwards, the Boss arrives for Angelica and notices Furuichi, mistaking him for Oga. He has guards surround the area outside Angelica's cell. Oga attempts to fight the thieves but suddenly, Baby Be'el starts to grow larger and larger due to being excited from the sight of Vlad's Master. He then grows to a giant-like size with Oga riding on his head. Oga notices that he and Vlad's Master are now equally challenged and orders the two to fight.

Baby Be'el and Vlad's Master start their fight. Baby Be'el ends up winning the match by utilizing most of Oga's regular fighting techniques, including the "Boston Crab" technique that Oga used in his battle with Hidetora Tōjō.

End of the Fight
At the same time, Furuichi has freed himself and tries to save Angelica who has been recaptured by the Thief Boss, despite not being a fighter. Baby Be'el steps on the Boss, narrowly missing Furuichi and Angelica and Lamia, causing Furuichi to start yelling at Oga. However, Oga appears before him and explains that he was thrown off by Be'el. Be'el continues to go on a rampage as he cannot stop his excitement and is about to stomp Oga and Lamia. Oga tries to stop Be'el but fails.

Much to his and the others' surprise, the same person who appeared in Angelica's house reappears to stop Be'el's attack. The stranger's name is Athrun. Athrun states that he wanted to know what a Royal Family contractor was like and tells Oga that he's disappointed in him. He then asks Oga to reverse Be'el's condition but Oga is ignorant of what he was saying, causing Athrun to get annoyed and try to kill Be'el. However, Oga then tells Be'el that it's time to eat, causing Be'el to revert to his original state. Athrun notes on how unusual the two are before leaving via another transdimensional stone. Before he leaves, he tells Oga that they'll meet again.

The thieves then separate into small groups and leave on their own. Oga and everyone free the other beasts and Angelica manages to convince Vlad's Master to return home. Lamia finally says her goodbyes to Be'el and, hesitantly, thanks Oga for helping her. Afterwards, Alaindelon appears after having recuperated, shocking everyone but Angelica. He sends Oga, Furuichi, and Baby Be'el back to the Human World where they arrive at the river. Hilda welcomes them home.

Meanwhile, Athrun returns to his master and tells of his experiences with the thieves. He states that he regained all 13 treasures that have been stolen, as well as meeting Be'el and his contractor, Oga. His master is impressed and Athrun asks whether he should have dealt with Oga, to which he is told that it didn't matter. Athrun is then told that he will meet Oga in the Human World, following Athrun's original words to Oga.

Afterwards, the summer then ends and a new semester begins at a new school.