Trying to phone a takeout restaurant, Keiichi Morisato, a student at the Nekomi Institute of Technology and member of the Auto Club, accidentally dials the "Goddess Helpline" and summons a goddess named Belldandy. Belldandy offers to grant him a single wish; Keiichi requests that Belldandy stay with him forever. The wish is approved, activating the "Ultimate Force," which prevents them from being separated. When they are driven out from the men-only dormitory, they search for lodgings for the night, but to no avail. Eventually, following Belldandy's lead, they reach an old temple. The next morning, Belldandy magically restores the temple to its former beauty before Keiichi awakens. Later that day, Keiichi's sister Megumi Morisato arrives and asks for a place to stay for a few days while she finds a permanent residence. The members of the Auto Club show up and a lively party takes place as the day ends.

Five months have passed since Keiichi and Belldandy began living together, but their relationship has not progressed. As summer vacation approaches, Keiichi (with prompting from Megumi) decides to take Belldandy to the ocean. While Belldandy and Megumi are shopping, Keiichi decides to watch a film that has mysteriously arrived in the mail. To Keiichi's surprise, Urd, Belldandy's elder sister, steps out of the television. They arrive at the beach and Urd reveals to Keiichi that she has secretly tagged along to help bring the couple together. However, Keiichi almost drowns after a boating accident and Urd's love potion causes Keiichi to fall in love with Sayoko Mishima by mistake. Urd's presence is revealed when Belldandy recognizes Urd's hand in the events, and a kiss from Belldandy brings Keiichi back to his senses. Urd's trip to Earth has caused problems in heaven. As punishment, she is forced to remain on Earth. She moves in with Keiichi and Belldandy.

Skuld, Belldandy's younger sister, is overworked from hunting bugs in heaven. A bored Urd continues to get in everyone's way on Earth by accidentally exploding a cake Belldandy is making and an engine Keiichi is repairing. Lonely and overworked, Skuld comes to Earth and tries to convince Belldandy to return with her. Keiichi is working hard to build a motorbike for an upcoming drag race. With Skuld's help, they complete the bike in spite of the difficulties posed by Toraichi Tamiya and Otaki Aoyama, leaders of the Auto Club. On the day of the race, Toshiyuki Aoshima, leader of a rival club, reveals that Belldandy is the prize for the race – the winning club will gain her as a member. Skuld, still hoping that Belldandy will leave Keiichi to return with her, tampers with the rival club's bike so that Keiichi cannot win. In the final race, Keiichi wins after taking the flag when the other bike's engine fails.

Keiichi has a nightmare about Belldandy. He wakes up to discover the ground inside the temple covered with snow; however, the ground outside is snow-free. Skuld identifies the problem as a bug and creates a bug-catching machine, setting everything back to normal. She notes that the bugs form when Belldandy and Keiichi are too close. The Almighty notifies Belldandy that she will be recalled in three days. When the couple travels to school, they do so separately, prompting rumors that they have split up. Once Belldandy tries to get closer to Keiichi, explosions cause chaos and Belldandy flees. Megumi suggests that Keiichi buy a ring for Belldandy, and so he works part-time jobs to earn money to pay for one. Urd and Skuld identify a cherry tree near the temple as the source of the bugs, but on informing Belldandy, Belldandy becomes upset.

Keiichi, unable to meet directly with Belldandy, apologizes and continues to work. Despite Urd and Skuld's efforts, they cannot find a way to remove the bugs created when Belldandy and Keiichi are too close. They resort to the Ultimate Magic Circle, a forbidden magic, as a solution. Keiichi eventually earns enough to purchase the ring. The sisters complete their research of the Ultimate Magic Circle. Belldandy, crying, removes all traces of herself from the temple and begins to depart through the gate. Keiichi arrives as she is departing and Urd casts her spell, closing the gate. Keiichi and the goddesses are thrown back in the ensuing explosion and Belldandy enters Keiichi's consciousness to save him. Keiichi's memories begin to return and he realizes that Belldandy made a forbidden promise, forcing her to seal all of Keiichi's memories. When he finally wakes up, The Almighty rescinds his order for Belldandy to return and Keiichi gives her the promised ring.