The story revolves around Issei Hyoudou, a lecherous high school student attending Kuoh Academy, a former all-girls school that has recently turned co-ed. He lives a peaceful life with his equally-perverted friends, Matsuda and Motohama, until he is called for a date by a girl named Yuuma Amano. At the conclusion of the date at night, Issei is brought to a local park, where Yuuma asks him if he wants to know what her wish is. Under the impression of receiving a kiss from the girl, Issei is shocked when Yuuma whispers the words "Can you die for me?" and transforms into a Fallen Angel. Issei is brutally attacked and killed by Yuuma, his last sight being a glimpse of red hair in front of him. The next day, Issei wakes up to find himself alive, leading him to assume the events that transpired yesterday were all but a bad dream. Later, after spending the night watching some porno DVDs with his friends, Issei walks home and finds himself in the same place where Yuuma had killed him. He is then attacked and wounded by a male Fallen Angel Dohnaseek until a crimson-haired woman named Rias Gremory steps in and saves him, just before Issei loses consciousness from the light spear that was thrown by the Fallen Angel. The next morning, Issei, under the assumption the events that occurred yesterday were all just a dream again, wakes up to find a naked Rias in his bed. Rias reveals her identity as a Devil to Issei, and claims that she has reincarnated him into a Devil, thus beginning his new life as a Devil servant in the Occult Research Club.
Ever since his girlfriend passed away, Mathias has had a hard time finding love, much less reciprocating it. When his friend Katy asks to be his girlfriend, things abruptly go sexual and Mathias is thrust into an uncomfortable situation where Katy is all over him.