GARO focuses on the life of Kouga Saejima, who assumed the title of Makai Knight to protect humanity against dark demonic manifestations called "Horrors."
In his quest to purge them, he encounters a young girl named Kaoru whom he saved from a Horror, but was stained with its demonic blood. As a rule, those that been stained by the blood of a Horror shall be cut down, or else they will die painfully in approximately 100 days. Kouga had mercy upon her and tries to find a way to purify her before her remaining time expires. Thus, the series focuses on Kouga's developing relationship with Kaoru and his stand in protecting humanity after the wishes of his father, who was his predecessor with the title of GARO.