Keiichi Morisato is a good-natured, yet hapless and girlfriend-less college freshman who is often imposed upon by his elder dorm-mates and brow-beaten into taking phone messages and doing miscellaneous chores for them. One day, while alone in his dorm, he accidentally calls the Goddess Technical Help Line and a beautiful goddess named Belldandy materializes in his room. She tells him that her agency has received a system request from him, so she has been sent to grant him a single wish. Skeptical and thinking someone is playing a practical joke on him, he wishes that she stay with him forever. To his surprise, his wish is granted. Belldandy must stay with him, but as his dormitory is strictly male-only, they are both forced onto the street.
They set off on his motorcycle to find alternative shelter, eventually seeking cover in an old Buddhist temple. In the morning, they are greeted by the temple's sole inhabitant, a young monk, who welcomes them and gives them permission to stay until they can find permanent lodging. He immediately puts them to work maintaining the temple grounds, but when he sees Belldandy use her powers to save Keiichi from injury, he begins to fear that she may be a demon or sorceress. He is eventually convinced of Belldandy's intrinsic goodness when he witnesses her solicitous care of the temple premises and her perfect meditation ritual. When he decides to go on a pilgrimage to India, the priest gives the couple permission to remain in the temple so long as they continue to maintain it.
The Oh My Goddess! universe is fashioned loosely around Celtic/Norse Mythology; various names and concepts are recycled for humor. Three worlds exist in the Universe of Oh My Goddess: Heaven, Hell, and Earth. Heaven is the realm of the All Mighty and goddesses, Hell is the realm of Hild and demons, and Earth is the realm of humans. Reality is controlled by an enormous and complex computer system, named Yggdrasil.