Tiel, the Beast Tamer and a member of the S-rank party "Garyu Hoshu," is one day expelled, deemed expendable.

Embarking on a new path as a solo adventurer alongside his partner, Pipi, a phoenix chick, Tiel discovers that Pipi, who he could never get to grow through any means, starts to grow by devouring monsters.

The now-grown Pipi evolves into an S-rank monster, a Phoenix. Thanks to this boon, Tiel acquires the skill "Kyozon Kyoei," which grants powerful upgrades every time he forms a contract with an S-rank monster.

To ally with powerful monsters he has yet to encounter across the world, Tiel decides to travel the globe.

Meanwhile, the Hero's party, now without Tiel, struggles with coordination, their wide-range attacks frequently resulting in friendly fire, and their former strength is a mere shadow of what it once was.

With Tiel's return to S-rank, the Heroes, stripped of the "Garyu Hoshu" name and demoted in rank, grow increasingly resentful towards him.

As he forms contracts with S-rank monsters one after another, Tiel realizes that he is approaching the status of the Demon Lord he once sought to defeat.

Aiming for the coexistence of monsters and humans, Tiel strives to transform the world as the current Demon Lord, and eventually finds himself confronting his former party leader, the Hero.