Ishiyama High is a school populated entirely by delinquents, where nonstop violence and lawlessness are the norm. However, there is one universally acknowledged rule—don't cross first year student Tatsumi Oga, Ishiyama's most vicious fighter. One day, Oga is by a riverbed when he encounters a man floating down the river. After being retrieved by Oga, the man splits down the middle to reveal a baby, which crawls onto Oga's back and immediately forms an attachment to him. Though he doesn't know it yet, this baby is named Kaiser de Emperana Beelzebub IV, or "Baby Beel" for short—the son of the Demon Lord! As if finding the future Lord of the Underworld isn't enough, Oga is also confronted by Hildegard, Beel's demon maid. Together they attempt to raise Baby Beel—although surrounded by juvenile delinquents and demonic powers, the two of them may be in for more of a challenge than they can imagine.

Private detective Sousuke Kaburaya is simply trying to make ends meet when Sara da Odin, an imperial princess from another world, falls through a portal right on top of him. Unaware that she is an anomaly in a magicless world, Sara uses powerful spells in public, causing a commotion and forcing Sousuke to take her back home with him. With the help of the internet, manga, and modern Japanese conveniences, Sara grows accustomed to living in Japan and becomes Sousuke's assistant in the process. Meanwhile, Livia de Udis, Sara's personal knight, also comes through the portal but lands at a different location. There, she meets a homeless man who readily accepts her unique background and odd tendencies. Despite her situation, Livia quickly adapts and settles into the homeless lifestyle, a far cry from the noble position she used to hold. While Sara and Livia may be living different lives, their positive and ambitious attitudes leave a lasting impact on the people around them as they experience both the good and bad of Japan.

The lives of high school freshmen Haruki Mishima and Towa Honda revolve around studying, clubs, and girls. Amid their hectic routines, romance is certainly budding for the two best friends. Since childhood, Haruki has been pining over Miharu Mashiki, and a chance encounter at the convenience store reignites his feelings. As for Towa, the class representative Mami Mihashi has caught his eye; but unfortunately for him, his affections are not reciprocated. While Haruki's more reserved approach to pursuing Miharu contrasts Towa's persistence with Mami, love seems to be elusive for both. As the months fly by, only time will tell if romance is in the cards for the duo.

High school student Taiyou Asano has been socially inept ever since his family died in a car crash. The only person he can properly interact with is his childhood friend, Mutsumi Yozakura—the head of the world's strongest family of spies. Mutsumi's eldest brother, Kyouichirou Yozakura, has been overprotective of her ever since he once rendered her severely injured. His love for Mutsumi is lethal, and Taiyou is his next target. To survive, Taiyou must marry Mutsumi and become a member of the Yozakura family. Thrown headfirst into chaos, Taiyou begins his journey to become a powerful spy in order to protect his wife and uncover the dark secrets of his past and the Yozakura family.