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.

In the wake of defeating Boros and his mighty army, Saitama has returned to his unremarkable everyday life in Z-City. However, unbeknownst to him, the number of monsters appearing is still continuously on the rise, putting a strain on the Hero Association’s resources. Their top executives decide on the bold move of recruiting hoodlums in order to help in their battle. But during the first meeting with these potential newcomers, a mysterious man calling himself Garou makes his appearance. Claiming to be a monster, he starts mercilessly attacking the crowd. The mysterious Garou continues his rampage against the Hero Association, crushing every hero he encounters. He turns out to be the legendary martial artist Silverfang’s best former disciple and seems driven by unknown motives. Regardless, this beast of a man seems unstoppable. Intrigued by this puzzling new foe and with an insatiable thirst for money, Saitama decides to seize the opportunity and joins the interesting martial arts competition. As the tournament commences and Garou continues his rampage, a new great menace reveals itself, threatening the entire human world. Could this finally be the earth-shattering catastrophe predicted by the great seer Madame Shibabawa? [Written by MAL Rewrite]