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.

After foiling a world-threatening plot, Shigeo "Mob" Kageyama returns to tackle the more exhausting aspects of his mundane life—starting with filling out his school's nerve-racking career form. Meanwhile, he continues to assist his mentor Arataka Reigen and the office's new recruit, Katsuya Serizawa, in solving paranormal cases of their clients. While continuing his duties, Mob also works on gaining more independence in his esper and human lives, as well as trying to integrate better with the people around him. However, new supernatural and ordinary challenges test Mob’s emotional stability and force him to confront the realities around him. As he strives to continue forward on the path to maturity, Mob must resolve his emotional crises and reassess the naivety he has held on for so long.

Hayase Nagatoro and Naoto Hachiouji have grown closer: the girl spends more time than ever in the art club room with her senpai. Although he is always on edge, Naoto no longer seems to mind Nagatoro's presence. Time and again, Naoto demonstrates his hidden, cool demeanor, and Nagatoro displays her possessive tendencies. However, they still can not seem to completely close the distance between them. It is clear to everyone else that the pair have feelings for each other. For Nagatoro, there is nothing more entertaining than toying with Naoto. But as the girl shows no plans to stop teasing her senpai, it is only a matter of time before they realize how they truly feel.

Koujirou moves to Tokyo from Japan's northernmost island of Hokkaido and insists on bringing his favorite horse, Hikarikin, with him. The alien king Rel arrives from planet Wedelun and gives him a belt that will allow him to transform into a Miracle Hero and to protect the world from alien menaces. He is more interested, however, in impressing the pretty Yuka, though his rival, Michinari, wants her for himself.