Set in the fictional realm of Tah'afajien, Not Gods But Monsters tells the story of Jehn Brumal, a teenager who is forced from her home when a private military force destroys the village of Vertegarte. Having lived a sheltered life, Jehn doesn't know who they are, or why they bring automaton soldiers, but she discovers that they are after her mentor, Marianus O'mas.

When she is eventually reunited with O'mas, a former spymaster and retired officer of the Verenigen military, the pair set off to meet his former associates from the war: gun-for-hire Zoe Agilis, strongman Zed Muntanya, scholar Flynn Earrele, archer Callie Evans and nihilistic assassin Kyote Kurttsen.

O'mas fears that industrialist Zane Grymore is stockpiling weapons for a potential coup. Even more troubling is Grymore's recent obsession with imbued ores, rare stones that have magical properties, and the Byrael, a pantheon of mystical deities said to have created mankind.

Jehn, O'mas and their allies are directed to an ancient ruin known as the Stairwell of the Byrael, where, it is said, man can commune with the gods. The truth they discover within the aeons-old structure reveals that Grymore's actions have the potential to destroy the world.