In this fanservice, harem series: All around pervert Issei Hyodo is a normal high school student who's never even had a girlfriend, until one day a beautiful stranger approaches him and asks him out. During their first date, she reveals herself as a fallen angel and slays him for the mystery "Gear" that dwells within his left arm, but as he lies dying, Issei is resurrected by his school's most beautiful girl; a stunning redhead by the name of Rias Gremory.
Rias reveals her true identity as a high-ranking Devil from the underworld, declaring that from this moment forward, he will be one of her servants. Now transformed into a Devil himself, Issei learns to adjust to his new lifestyle while battling Fallen Angels and rival Devils for the fate of all creation. Along the way, the two develop feelings for each other as several other girls compete for his affections.