Listless geek Mizuto Irido and introverted nerd Yume Ayai seemed like a match made in heaven, connected by their mutual love for literature. Unfortunately, their differences gradually grew, and they separated just after their middle school graduation. But, as if by divine comedy, the two find themselves reunited as step-siblings. A rivalry begins to brew between the former couple, both unwilling to acknowledge the other as the older sibling. In an attempt to "solve" this issue, Mizuto and Yume agree upon a rule: whoever crosses the boundaries of siblinghood norms loses, and the winner will not only be called the older sibling, but also get to make a request. However, now that they live under the same roof, the lingering memories they share start to influence their actions—possibly rekindling the feelings that may not have been fully extinguished in the first place.

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.

World-class medical researcher Kanji Yakutani lost his little sister to a tumor years ago due to ineffective treatment. To honor her legacy, he has dedicated his research to developing new medications for such conditions. But as fate would have it, he overworks himself and passes away at the age of 31, only to be given a second chance at life in another world. When he wakes up, Kanji finds himself in the body of a 10-year-old boy named Falma de Médicis, the son of an esteemed family of medical practitioners in the Sain Fleuve Empire. Bearing the mark of a deity's divine blessing, Falma is capable of performing a unique divine art, allowing him to create and reduce any substance with the knowledge of its chemical properties. After reading through some pharmacology books, Falma realizes that this world is operating on similar medical practices as in ancient times. He also learns that medicine is an exclusive privilege to the nobility, depriving commoners of proper medical care. Using the knowledge from his past life and the divine abilities granted to him, Falma resolves to make medicine available to those who need it—irrespective of class.

Due to a tragic accident, Banri Tada is struck with amnesia, dissolving the memories of his hometown and past. However, after befriending Mitsuo Yanagisawa, he decides to move on and begin a new life at law school in Tokyo. But just as he is beginning to adjust to his college life, the beautiful Kouko Kaga dramatically barges into Banri's life, and their chance meeting marks the beginning of an unforgettable year. After having a glimpse of college life, Banri learns that he is in a new place and a new world—a place where he can be reborn, have new friends, fall in love, make mistakes, and grow. And as he begins to discover who he was, the path he has chosen leads him towards a blindingly bright life that he will never want to forget. [Written by MAL Rewrite]