When he was in his third year of middle school, Nisaki Shio was summoned to another world as a hero. He succeeded in defeating the Demon King, saving that world, but was forcibly returned to modern Japan without any reward. Now three years have passed, and Shio has done his best to move on from his traumatic experiences, graduating from high school with the support of those around him.

Shio is suddenly summoned back to the same other world by a woman claiming to be a goddess. The world is once again in peril, this time due to the rampage of a mysterious secret society calling itself Helheim. To his utter horror and embarrassment, Shio discovers that Helheim is the cringeworthy organization he himself created during his chuunibyou phase, a period when he called himself the King of Corpses and styled himself as a dark edgelord. Someone is now impersonating his creation and tarnishing its reputation, using Helheim to spread chaos across the land.

Burdened by the responsibility of dealing with the monster of his own making, Shio sets out to stop Helheim and clear its name. To accomplish this, he must reassemble the eight former executives who once served under him, known as the Eight Precepts, who were the core members that originally made Helheim the strongest organization in the world. As he reunites with each of these former subordinates, he writhes in embarrassment at his past selfs enthusiasm while reluctantly slipping back into the role of the Corpse King.

The journey begins in the steel city of Gagiul, a struggling industrial center facing famine and political turmoil. There, Shio must rescue a countess described as being of dull colors who stands in peril, demonstrating that his mission will require both combat prowess and leadership. As he travels across the world, he must solve the mystery of who is actually controlling Helheim and why someone would resurrect his embarrassing dark history for real. Along the way, he confronts the gap between the person he was as a delusional middle schooler and the person he has become, all while trying to save a world that cannot seem to survive without him.