During what should have been an ordinary day at U.A. High, a group of students and heroes are suddenly transported through an unexplained phenomenon and thrown into a completely different world. There are no modern cities, no hero agencies, and no explanation for how they arrived. Instead, they find themselves in a kingdom filled with royalty, political tension, magic, and an ongoing conflict between humans and the magic-wielding Sergos.

Their sudden appearance immediately causes suspicion.

To the people of Calaveras, Quirks look dangerously similar to unfamiliar forms of magic. To the MHA characters, the Sergos and their abilities are equally confusing. With neither group understanding the other's world, the arrival of several powerful strangers quickly becomes a problem the royal family cannot ignore.

At the center of it all is Prince Osora.

Already struggling beneath the expectations placed upon him as Calaveras's future ruler, Osora suddenly finds himself dealing with a group of people who come from a world where someone can become a hero regardless of whether they were born into royalty. Their existence introduces an entirely unfamiliar idea into his life: a world where identity and one's future might not be decided solely by family, status, or tradition.

Meanwhile, Arias remains deeply involved in Osora's life and immediately becomes suspicious of the newcomers, especially when their strange abilities begin attracting unwanted attention. Catalina and Celia find themselves equally curious about these visitors, while the larger conflict involving the Sergos makes their arrival far more dangerous than anyone initially realizes.

As Class 1-A and the heroes attempt to find a way home, they become caught in the complicated lives of the people of Calaveras. The crossover follows both casts as they navigate cultural differences, magic and Quirks, political conflict, royal expectations, friendships, arguments, and the growing danger surrounding Osora.