Wataru Sajou is infamous in his school for persistently seeking the attention of his crush, Aika Natsukawa. His usual day revolves around shadowing her and confessing his undying love at every turn. However, a moment of epiphany leads Wataru to relinquish his childish behavior and face the reality that she will never return his one-sided feelings. He distances himself from Aika and spends his newfound spare time with his friends and in the company of other lovely girls. As Wataru moves on, Aika starts approaching him in unusually suggestive ways—making him question whether giving up on his love was the right decision after all.

On Christmas Eve, four-year-old Miri Unasaka arrives in Tokyo completely alone, in search of her father. The bright lights and merry atmosphere guide Miri to a big hotel and a man with a delicious-looking cake. However, the child has just unknowingly walked into the center of an elaborate, foolproof plan for assassinating a dangerous mafia boss. Professional assassins Kazuki Kurusu and Rei Suwa live together, fleeing memories of their grim childhoods and avoiding emotional connections. When their mission goes awry and they end up bringing Miri home, there is only one reasonable thing to do: return Miri to her mother. But the girl's innocent laugh and pure worldview quickly enamor her to Kazuki, who secretly enjoys playing a parent, and it is not long before Rei's impenetrable heart makes room for Miri. Miri's every move is unpredictable, and Kazuki and Rei find raising an energetic child harder than any of their missions. Although it presents a great risk to their careers, Kazuki and Rei—both assuming the affectionate nickname "papa"—decide to provide Miri with a normal childhood despite all the odds stacked against them.

Since the death of his parents, Shinpei Ajiro had lived with the Kofune family and their two daughters—Mio and Ushio. Although he then left his home island to continue his education in Tokyo, Shinpei returns after Ushio tragically drowns during the attempted rescue of a little girl. During the funeral, his best friend informs him about bruises found around Ushio's neck, casting doubt over the cause of her death. Suspecting a murder has taken place, Shinpei reevaluates recent events, but strange incidents only continue to transpire. Disappearing people and other unexplainable occurrences lead Mio to recall an old folktale referring to entities called "Shadows," which may not be entirely fantasy. Supposedly, an encounter with one's Shadow foretells the person's impending demise. Facing the dark side of Hitogashima Island, Shinpei stands against his grim fate to fulfill Ushio's final will—to protect Mio.

The "God of High School" tournament has begun, seeking out the greatest fighter among Korean high school students! All martial arts styles, weapons, means, and methods of attaining victory are permitted. The prize? One wish for anything desired by the winner. Taekwondo expert Jin Mo-Ri is invited to participate in the competition. There he befriends karate specialist Han Dae-Wi and swordswoman Yu Mi-Ra, who both have entered for their own personal reasons. Mo-Ri knows that no opponent will be the same and that the matches will be the most ruthless he has ever fought in his life. But instead of being worried, this prospect excites him beyond belief. A secret lies beneath the facade of a transparent test of combat prowess the tournament claims to be—one that has Korean political candidate Park Mu-Jin watching every fight with expectant, hungry eyes. Mo-Ri, Dae-Wi, and Mi-Ra are about to discover what it really means to become the God of High School. [Written by MAL Rewrite]