Alvin, possessing top-tier abilities, wants nothing more than an easy life, so he's been pretending to be incompetent. However, his doting older sister (who even proposed marriage to him!) accidentally witnesses his true power! "I knew it! I believed in you, little brother! I have to go tell everyone right away!" The rumor spreads like wildfire, and Alvin's treatment at the academy completely changes! He instantly defeats students who dare to challenge him, is recognized as the strongest mage by the princess (who also happens to be the knight commander), unexpectedly rises through the ranks of the knight order, and somehow finds himself surrounded by more and more girls—where did all the peace and quiet go!? "You're so cool! I think I’m falling even more in love with you, little brother!" ...Meanwhile, completely oblivious to Alvin's anxieties, his sister gives him a huge hug.

Rion, a boy who has survived for ten years in the "Demon Realm," a land overrun with monsters, is secretly a former prince, banished for being a cursed child without a crest. Though he had vowed never to involve himself with the royal family again, Rion learns of an assassination attempt on his younger sister, Princess Celestia, and finally decides to return to the capital. Concealing his true identity, Rion reunites with his sister, acting as her bodyguard at times, and a mysterious swordsman at others, eliminating the threats against her one after another. When an unprecedented crisis, one that threatens the entire kingdom, befalls the princess, Rion unleashes the true power he has kept hidden—the power of the Heroic King that dwells within him! A clandestine, unrivaled fantasy of the strongest prince, inheriting the power of legendary kings!