The earth is plagued by "Witches," monstrous creatures who devour human beings. An organization called Umbella was established in an effort to combat the problem's growth. 
Umbella trains young and talented investigators, known as "Erasers," in the ways of detecting and disposing of these beings. 
Erasers normally work in teams of two, with the other half being a human that can turn into a weapon called a "Relic". 
During the training period, Ayato Amaya and Aviorah Himeragi are partnered up and thus begin their investigations.