Rein is a beast tamer who was expelled from the hero's party who consider him too weak because he only has the ability to form contract with animals. While looking for work, Rein rescues Kanade, a member of the cat tribe who becomes his companion. By forming a contract with a member from one of the strongest races, Rein obtains some of her powers, becoming much stronger. The two then begin their journey together, with Rain befriending and taming other girls from different races who become powerful companions while obtaining new abilities with each new contract formed.
