From the introduction:
The magical world of Demons and the modern world of Humans, two sides of the same coin that weren’t supposed to ever fuse. However, the gate separating these two worlds has been weakening continuously, thus leading to certain mix between these worlds. As such, the ‘Monster Tamers’, special humans with natural boons, are the keepers of peace by making contracts with monsters to help them maintain things in order, while keeping things hidden from most humans. For demons, this partnership grants several advantages, for Tamers, it means the extra legwork needed to do their job.