Years ago, the opening of an interdimensional portal released two beasts into the skies over the Sea of Japan, visible to all the countries along the coast. After destroying the naval patrol sent to investigate the disturbance, the two beasts then turned against each other. Witnessing the fight from Japan, scientists Joe and Marie Hayakawa were sucked into the rift. Upon returning to Earth, they compiled and published the academic findings from their voyage in a legendary book titled ''Day of Succession''. Attempting a second expedition to that other dimension twelve years later, the couple have not returned. Twins Ai and Yu Hayakawa decide to set forth in search of their parents and the mysterious Wonderland they studied. In the ruins of an abandoned subway station, the two wait for a train to take them to the world beyond. On board they meet Lisa Pacifist, who quickly decides to aid the two in the search for their missing family. Can the three of them manage to find Joe and Marie?

Miyagawa Atsushi is an 11-year-old boy who is 180 centimeters tall. Since he has a grown-up looking build but acts his age, he has always been running into troubles. On the other hand, his 17-year-old elder sister Atsumi is only 137 centimeters tall. Even though she has a childish looking build, Atsumi takes good care of Atsushi.

In the Tranquil Sun province of the empire, there exists an unremarkable lordship known as Xue Ying Territory! This is the home of the Dong Bo clan, the clan of our hero Xue Ying! His father, a commoner turned noble; his mother, a noble who abandoned her clan for love, and his brother, an innocent toddler. But peace cannot last forever, Xue Ying’s peaceful life is shattered, and the only way to reclaim it is through power! (Source: Novel Updates)

The four princes of Grannzreich—Kai, Bruno, Leonhard, and Licht— continue to compete for the right to their kingdom's throne under the guidance of their beloved royal tutor, the competent yet childlike Heine Wittgenstein. One day, during a political visit to Grannzreich, the king of the neighboring Romano Kingdom arrives at the royal palace with his twin sons, Ivan and Eugene. In the hopes of befriending the twins, Heine and the four princes meet their visitors, but their efforts are thwarted instantly when the two twin princes declare that they refuse to address those who will never reach the throne. Dispersing the tension between the boys, Heine reveals that he, upon their father King Victor von Grannzreich's request, will be tutoring the Romano princes alongside the Grannzreichs for the duration of their stay, in hopes of strengthening the future relations between the two kingdoms. [Written by MAL Rewrite]