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.

Keiichi Morisato

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 Norse mythology; various names and concepts are recycled for humor. Three worlds are presented -- Heaven, Hell, and Earth. It seems apparent that many other worlds also exist. Heaven is the realm of the Lord and goddesses, Hell is the realm of Daimakaicho and demons, and Earth is the realm of humans. Reality is controlled by an enormous and complex computer system, named Yggdrasil.

Each goddess is assigned Class, Category, and Limitations. Class indicates power and skill in performing pure magic. Only First Class (better) and Second Class (lesser) are cited. First Class goddesses are held to a stricter standard regarding the prohibition to lie. Three Categories are cited: First, which is Administration, Second, which is Commercial (deals with customers), and Special Duty, which includes the Combat Division. Limited/Unlimited indicates boundaries on permitted actions. Goddesses may be penalized, even having their license suspended for a time, for dereliction of duty. A goddess using her powers during suspension will have her license permanently revoked. Belldandy is a Goddess First Class, Second Category, Unlimited License. Urd is a Goddess Second Class, First Category, Limited License. Skuld is a Goddess Second Class, First Category, Limited License.

Due to safety concerns regarding their enormous powers, goddesses wear power limiters, usually in the form of jewelry, such as earrings or bracelets. It is said that a Goddess First Class at full power could easily destroy the Earth, if not careful. Belldandy's power limiter is the second clasp on her left ear.

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The three main goddesses in Heaven

Goddesses also possess particular proficiencies in certain magical domains: Belldandy often uses wind magic and transports via mirrors, Urd uses electricity and television screens, and Skuld uses water. Urd, Belldandy, and Skuld are respectively associated with being the goddess of the Past, Present, and Future. Although their primary power and energy comes from Heaven, they can also recharge in various ways. Typically, Belldandy can accomplish this by sleeping, Urd by drinking Sake, and Skuld by eating ice cream.

The lifespan of goddesses seems unclear. While they have always been alive and will always be alive, it seems they are reincarnated at some point. Belldandy was fully a child when she first met Keiichi. Also, goddesses and demons have an agreement in place to dissuade each one from killing the other. Old age for goddesses has not been demonstrated in the series, so their elderly years are unknown. The goddesses' earthly metabolism can vary from humans and from each other as well. Belldandy, for example, becomes drunk on cola, but Urd and Skuld are not affected at all.

The goddesses' purpose is to bring happiness to everyone around them. Toward that end, heaven has created the Goddess Technical Help Line (also called Goddess Relief Agency), designed to bring happiness to the people of Earth, especially those with great virtue but terrible misfortune. A competing institution, named the Earth Assistance Center, also is staffed by goddesses. In most scenarios, a goddess appears before one that the system has deemed worthy and grants him or her one wish. The wish must be approved by the system, after which a contract is created between the human and the goddess and stored on the Yggdrasil system as a file. The wish contract file is protected by a pass code known to the Goddess. As demons work toward the opposite end, the total happiness on Earth must remain in balance. Heaven and Hell strictly abide by an agreement to work through contracts and never kill each other.

Demons have similar class and license restrictions, and are accompanied by familiars instead of angels. A seal exists between the demon world and Earth, named the Gate to the Netherworld. It was "created by the gods and can only be broken by an instrument of the gods." As such, demons require a catalyst to manifest on earth. The demons possess a system similar to Yggdrasil, named Nidhegg. Both demons and goddesses possess the power to seal beings away. The demons also operate in a fashion similar to goddesses by creating contracts with humans and offering them wishes, but often at a price.

Other elements in the Goddess universe are programs that can be constructed by either the goddesses or the demons for any number of purposes. One example is the Ultimate Destruction Program, a program written by demons to create a world for demons by destroying humanity. Countermeasures include directly destroying the program or using a vaccine, a program specifically designed to nullify its target program. Programs designed to destroy systems and with self-replicative abilities are known as viruses, much like the computer viruses in the modern world.

A goddess does not have an angel automatically, but receives one in egg form. When her power matures, the egg hatches into an angel, becoming a lifetime companion. The angel always obeys the goddess, being a reflection of the goddess' inner self.

Other creatures that exist in the Earth plane are a multitude of spirits that are responsible for almost every aspect of life. These include the spirits of Money, Wind, Engine and such. More specific entities include Earth spirits, which are guardians over a specific area of land. Morgan le Fay, a villain from the movie, is probably a high ranking Earth spirit (or a being from another dimension, but that is less likely because her tragic love story with a human must have happened on Earth) who demonstrates great strength fighting Belldandy and Urd, even though her powers are less potent than the ones of goddesses of their level.

As the manga's popularity increased, a five-episode original video animation (OVA) was made based on it. With the OVA's success, the stage was set for two TV series and a film to follow.