In this world there are mysterious machines called "Tremble." No one knows where they came and why they were built. The only thing known is that they continue to evolve and attack humans. The only way to defend yourself is to try to destroy them, but there are those who have learned to control certain and use them to your advantage ... the Breeder. These extraordinary individuals, however, have a difficult life, always on the run from people who fear them and treats them as criminals. But Ion is not one to be discouraged, so he decided to follow his father's footsteps and become a Breeder, a tamer of Tremble!

(Source: Anime Click)