In an alternate Earth, where everyone fights monsters that appear from portals ever since WW1, a teen named Keaton Ark is just your average government-sponsored monster hunter despite his parents' wishes. Yet, while he makes a good profit from killing monsters, he knows time is running out for him, unless you're truly a hero amongst hunters you won't last very long. However, when he finds a mysterious store by the name of the Collectanea, which appears to be a simple bookstore that sells fables, it turns out to be far more useful than expected.