In a fantasy world where humanity lives behind fortified walls, monsters known as demons roam the lands and periodically surge in great waves called Calamities. The only ones capable of fighting these creatures are adventurers who absorb demonic flesh to gain their powers, a practice that slowly transforms the consumer into something less than human. The protagonist, Laevatein, is a young man who was once a celebrated hero but now lives as a disgraced outcast. Years earlier, during a catastrophic battle, he was forced to eat the heart of a powerful demon lord to save his comrades, an act that granted him immense strength but also cursed him with a ravenous hunger for monster flesh that he can never fully satisfy. That hunger, combined with the monstrous physical changes he undergoes when fighting, has made him feared and reviled by the very people he protects.

The story begins with Laevatein working as a low-ranked monster eater, a specialized adventurer who consumes demon parts to temporarily boost his abilities, but he hides the true extent of his curse. He is hired by a young noblewoman named Filia, whose village is threatened by an approaching Calamity. Unlike others, Filia does not recoil from Laevatein’s nature and instead sees his power as their only hope. As they travel together, Laevatein must balance his duty to protect the innocent with the ever-present danger of losing himself to the insatiable hunger that grows stronger with each demon he devours. The narrative follows their journey across the frontier, where they encounter other monster eaters at various stages of corruption, some who have embraced their monstrous sides and others who desperately cling to their remaining humanity.

A major narrative arc involves the revelation that the demon lord whose heart Laevatein consumed was not a mindless beast but a sentient being with its own memories and will, which now slowly merge with Laevatein’s consciousness. This internal conflict forces him to question whether the demons are simply enemies to be eradicated or if they are another race with their own civilization and reasons for the Calamities. Another significant storyline focuses on the Adventurers Guild’s secret experiments to create artificial monster eaters, using children as test subjects, which leads Laevatein to confront the human institutions that once hailed him as a hero. The manga also explores the backstory of Laevatein’s former party, who betrayed him out of fear after his transformation, and their eventual reappearance as either allies or enemies depending on their own paths of corruption.

As the Calamity grows closer, Laevatein must make a critical choice: fully surrender to the demonic hunger to gain enough power to save Filia’s village, or find a way to suppress it and risk defeat. The narrative culminates in a battle that forces him to merge with the demon lord’s personality, creating a third identity that is neither fully human nor fully demon, but something new that can communicate with the approaching horde. The series ends with Laevatein setting out alone into demon territory, not as a hunter, but as a diplomat seeking to understand the source of the Calamities, leaving Filia and the human world behind, but with the promise that he will find a way to end the cycle of eating and being eaten.