Nekomi Institute student Keiichi Morisato is ordered by his sempai (seniors) to stay at the club dorm on a Saturday night to take and return calls. A nice guy that people take advantage of, he does as ordered. But when he tries to call his sempai, he dials a wrong number...and his world is changed forever.
When a mysterious young woman flies out of his mirror after he dials a wrong number, Keiichi Morisato discovers that he has been granted one single wish by Heaven in compensation for his life of troubles and problems. The young woman, the Goddess Belldandy, explains the rules of the wish and why the skeptical Keiichi should believe in her abilities to grant it. But neither of them are prepared for the wish he makes, and the consequences that grow from it in plenty.
With the full scope of his wish now in play, Keiichi and Belldandy must find a place to live that a poor college student and innocent Goddess can afford. When Belldandy uses her powers to assist, it costs Keiichi his last bit of cash but leads them to a place neither could have guessed would be shelter...and Belldandy reveals a hidden part of herself in the process.
Now that they are settled in, Keiichi has to return to school...and Belldandy asks to come along. However, Campus Queen Sayoko Mishima sees the beautiful Belldandy as severe and absolute competition that she must crush and destroy...even if it means stealing Keiichi away from Belldandy!
Settled into their new home and settling in at school, Keiichi realizes that he has not been able to express his growing feeling of romance for Belldandy. Afraid that his shyness towards beautiful women and Belldandy in particular will cause her to lose interest, Keiichi seeks a way to declare his love for her...if only he can overcome that inability to express himself in a forthright manner!
A young woman hitchhikes from Hokkaido to see Keiichi Morisato, only to discover that he is living with the "beautiful foreigner", Belldandy. It is his sister, Megumi, who has come ahead of her graduation to find a place to live when she goes to school the following month. The search is arduous, but when the place she finds is too good to be true...it is, and the Goddess Belldandy must take control of the situation.
Innocent of Earthly and Japanese customs, Belldandy is educated on the White Day/Valentine's Day tradition by Keiichi's sister, Megumi. However, when it comes time to give Keiichi his gift, Belldandy finds that she doesn't know what a "romantic setting" is. Unfortunately, jealous Sayoko offers to "help" by introducing Belldandy to her cousin, wealthy playboy Aoshima Toshyuki...who is more than willing to show Belldandy what a "romantic setting" is in his own mind. Aoshima, however, finds out what it means to force himself on a Goddess First Class, Unlimited Category!
The holy contract between Keiichi and Belldandy is unprecedented and unique, but there are those in Heaven who want to see it kept as well as those who want to see it broken...because the contract has caused the Heavenly mainframe, Yggdrassil, to crash and cause havoc in Heaven and Earth...and Belldandy has become ill for the first time in her knowledge. As a result, an emissary from Heaven comes to "assist"...only she does so without permission and with her own private agenda for Keiichi and Belldandy, starting with an otherwise innocent looking videotape!
Now under one roof with two Goddesss, Keiichi Morisato finds it difficult to ride herd on both in order to prevent people from discovering their powers. And it doesn't help matters when both Sayoko and Aoshima, who have seen their power at work, decide to expose them to the world!
While the Goddesses hold the CD that can trap her in Limbo, First Class Demon Marller finds a human to make a deal with, letting her defeat the Goddesses and fulfill her mission on Earth at the same time. And when Sayoko says yes to Marller, Keiichi finds himself torn from his feelings for Belldandy and paying more attention to Sayoko. Will Keiichi revoke the holy contract by saying he no longer wants Belldandy by his side forever, and send her back to Heaven? Not if big sister Urd has anything to say about it!
When Heaven's main computer, Yggdrassil, crashes due to an infestation of 'bugs', a lone Goddess determines that she needs the help of her big sister...Belldandy! However, that help would force the breaking of the holy contract between Belldandy and Keiichi, and Belldandy would return to Heaven...forever! Urd, however, suspects that her youngest sister may actually have another agenda in mind.
Young Goddess Skuld tries to adapt to life on Earth...not! She only wants to convince Belldandy to leave Keiichi and be with her in Heaven always. Jealous of him and his hold on Keiichi, Skuld tries to prove her value to Belldandy...only to run into Keiichi's sister, Megumi, who becomes her arch-nemesis for questioning Skuld's mechanical abilities, culminating in a robot war that splits the Nekomi Tech campus!
Skuld still can't accept sharing her sister Belldandy with anyone, let alone Keiichi. In an effort to prove herself more worthy than him, Skuld decides to build a labor-saving device that will serve and protect Belldandy from harm, the First Class Demon Mara, and especially from Keiichi! However, Skuld is a more skillful mechanical genius than she thought, and the labor-saving device begins reprogramming itself!