The story follows the "strongest juvenile delinquent," Tatsumi Oga, a first year at Ishiyama High, the school for delinquents. One day while sleeping next to a river, he sees a man floating down it. Tatsumi pulls him to shore, and the man splits in half revealing a baby boy. This boy is the son of the demon king, and Tatsumi has been chosen as the one to raise it with the baby's demon maid, Hilda. The story follows his life with the child at the delinquent school.
(Source: Wikipedia, edited)