While on the road to their field trip, Class 2-C's bus is enveloped in light, and the Goddess, Vicius, summons the students and their teacher to another world. The Goddess hopes to raise them as Heroes to defeat the Great Demon Empire, and she welcomes the students after appraising their abilities with her crystal. However, Mimori Touka is appraised as an E-Class, the lowest rank... and he's branded a trash hero.

Touka rediscovers his cruelty as he kills monsters in the Ruins of Disposal. He means to escape and exact his revenge on the Goddess Vicius, but to get there, he’ll have to face the grotesque monster that’s killed so many before him—the Soul Eater.

Now that he's armed with experience and a book of forbidden magic, Touka sets out on a journey for revenge. But before he can get far, he encounters a beautiful woman fleeing a group of four men known as the Holy Watchers.

Touka arrives in a new city and decides to try his hand at the local ruins in hopes of scavenging material for a special potion to enhance his pet slime. Little does he know he'll find far more than he intended to down there.