In 2000, Ah! My Goddess: The Movie (劇場版「ああっ女神さまっ」, Gekijouban Aa! Megami-sama) was produced by Kodansha, Dentsu, Sega Enterprises, Pony Canyon, Nippon Shupan Hanbai, Anime International Company, MOVIC and Shochiku, featuring the entire cast of the Oh My Goddess! series. The animation was produced by AIC. It was also distributed by Shochiku in Japan and Geneon Entertainment when it was licensed in the US in 2001. Though the main plot is a canon of its own, many of the plot elements and side-events are based heavily on elements from a wide variety of chapters from Kousuke Fujishima's original manga.

A fairy princess, Morgan le Fay, crosses the moon, heading to the Lunar Prison. There she works on releasing the seal on the prison, despite the pain the barrier inflicts upon her. Once the seal is released, she is able to make contact with a small mask that reveals that they must unite for their common goal.

It is the start of the New Year at Nekomi Institute of Technology, and the various clubs seek to recruit new members. The NIT Motor Cycle Club is no exception, and tries to lure new applicants with a display of the bikes they race with; including the new two-seater Keiichi Morisato and Belldandy shall be using in an up and coming race. Many new members initially join, including Morgan, but most are soon scared away by Toraichi Tamiya and Otaki Aoyama.

That night, the old club members and the few new members celebrate with a party. Seeing Sora Hasegawa and Megumi Morisato drunkedly hanging off Keiichi incites a burst of jealousy in Belldandy that manifests itself by exploding every glass and bottle in the room. Mortified by her loss of control, Belldandy runs off into the night, with Keiichi in pursuit. When Keiichi catches up to her, she apologizes; characteristically, Keiichi simply smiles and comments upon the arrival of spring. Comforted, Belldandy smiles back and tells him amidst a flurry of cherry blossoms that she wishes she would spend all of her springs with him.

It is at this point that a mysterious figure reveals himself by walking into the moonlight. Recognizing him, Belldandy runs into his open arms and cries tears of joy. She introduces the man to Keiichi as Celestin, the man who acted as a mentor and instructor to her in her youth. Urd flies in to chase Celestin away, angrily ordering him to step away from a confused Belldandy. Instead, he turns Belldandy around and kisses her mouth, much to everyone's shock, whereupon she collapses. Alarmed, Urd attacks him with a bolt of lightning, but it is too late; he has vanished, and Belldandy lies unconscious in Keiichi's arms. Back home, Urd finds that Belldandy has been infected with a virus. What's worse, Peorth calls to inform Urd that Yggdrasil has also been compromised by the virus, and isolates The Heavens from Earth as a security measure until it is stopped, meaning that Belldandy cannot receive treatment until the matter is resolved.

When she finally regains consciousness, Belldandy appears to be perfectly healthy, with one glaring exception: she can recall neither Keiichi nor the time she has spent on Earth with him, even though she can identify Skuld and Urd immediately. Urd recognizes the symptoms as selective amnesia and informs Keiichi that all of Belldandy's memories after their first encounter must have been sealed. Proving Urd's speculation, Belldandy informs Keiichi that she may grant him one wish. After Skuld reprimands him, Keiichi craftily wishes that Belldandy's memories be restored, but as Yggdrasil is down, Belldandy finds that she cannot process the request. Skuld attempts to restore Belldandy's memory with a number of inventions, but the most they do is remind Belldandy to give Keiichi her business card. Unable to find an immediate solution, Keiichi decides to accept the current situation for the moment, and try to live as normally as possible.

When the NIT MCC are told about Belldandy's condition, they are shocked, but also concerned about their up-and-coming race; the competition is a mixed-gender race. When Sora declines the opportunity, Morgan offers to take Belldandy's place. Unsure about her skill, the MCC give her a trial-run with Keiichi; seeing them ride the bike triggers some traces of memory to return to Belldandy.

Up to this point, Keiichi has managed to remain surprisingly stoic despite Belldandy's condition. However, as he drives home that night at dangerously high speeds, he is so immersed in his conflicted mind that he subconsciously expects Belldandy to aid him in making a sharp turn, forgetting that those memories remain locked. Sensing imminent danger, Belldandy uses her powers to extract them both directly before the crash. Seeing Keiichi's guilt stirs something in Belldandy's heart, perhaps a faint reminder of the love they once shared.

When Belldandy accidentally uncovers a photo album filled with pictures of the two, she realizes just how deeply her amnesia has affected Keiichi. Seeing the recent emptiness in his smiles inspires Belldandy to get to know him better, starting by opting to remain as Keiichi's partner in the upcoming race despite her amnesia. When an irritated Morgan hears about Belldandy's renewed resolve, she challenges Keiichi and Belldandy to a race, teaming up with Megumi at Keiichi's request. Despite their best efforts, Megumi and Morgan are no match for the couple, and what's more, the experience apparently unlocks more of Belldandy's memories.

That night, Belldandy accidentally overhears a discussion between Keiichi and her sisters regarding Celestine's role in the current crisis. Realizing that Celestine erased her memories and inserted the virus, Belldandy stumbles out into the night, shocked and confused. Taking advantage of the situation, Celestine lures Belldandy away, disguised as a pulse of light. When she finally confronts him, he forcibly unlocks the memories of him that were sealed away by the gods. When Urd arrives to intervene, Celestine uses the virus to turn a senseless Belldandy against her elder sister in combat. Keiichi's timely arrival ignites a massive flux of uncontrollable energy from Belldandy as she struggles to reaffirm her sense of self, knocking out everyone in the vicinity. Successfully finding the side of her that she hid away after Celestine's betrayal allows her to safely dissipate the energy, though she faints in the process. Skuld wakes up to find the locality ravaged in the aftermath of the battlefield. When she spots Celestine calmly stepping down the stairs, she is unable to control her emotions, unwittingly summoning Noble Scarlet, her angel, and knocking Celestine away with an unrestrained bolt of magic that manifests as a deluge of water (true to Noble Scarlet's affiliation with the water element) that roars through the park. Keiichi awakens just in time to notice the oncoming wave, and rushes to cover Belldandy, so that he will take the brunt of the blow for her. Belldandy awakens to find Keiichi comatose on the ground, and hysterically tries to revive him.

After the battle, Belldandy comes to her senses, but her heart is uneasy. Tearily, she believes that she has only served to bring suffering to Keiichi. Skuld assures her that, regardless of present circumstances, in all of the friends they have made and all the experiences they have been through, Keiichi and Belldandy have weathered all with smiles. Reassured, Belldandy falls into Skuld's lap, sparking concern from Skuld, but Urd assures her that Belldandy is merely exhausted from the battle.

When Keiichi walks into the empty MCC clubhouse alone the next morning, he is alarmed to find a trail of blood leading to Morgan on the stairway. Keiichi insists that she come with him to the hospital wing, despite her assurances claiming otherwise. While he treats the gash on her arm, Morgan cryptically asks him whether he could love her, telling him that Belldandy will be "Celestine's girl." Before he can reply, she forcibly kisses him; unbeknownst to them, Belldandy sees them through the door. When her heart is ready to break, seeing Morgan and Keiichi kissing, she prepares herself to accept the vaccine, which will destroy the virus in her, at the great risk of destroying all her memories. Peorth administers the vaccine, but something goes horribly wrong.

The virus inside Belldandy has been using her as a Trojan horse; since Goddesses are connected to Yggdrasil, it was able to infect the main system through her, although without direct access it was only able to make slow progress. Heaven made direct access with Belldandy in order to administer the vaccine, unwittingly allowing it even deeper into their system. More seriously, while usurping this direct link, the virus somehow rewrites itself using the vaccine, turning itself into a program that bypasses all of Yggdrasil's security measures. Exploiting the direct link between Belldandy and Yggdrasill, Celestin hacks deep into the system mainframe, accessing an enigmatic program that to this point is referred to only as "top-secret." He pulls the program through Belldandy, and incorporates it into the vaccine mandala, creating an enormous magical field that swallows the temple and the surrounding forest. Three enormous tree trunks spiral from the ground into the sky, and a gargantuan being slowly materializes.

Casting back to the past, we see how Celestin, concerned that the Gods were not listening to the suffering of those of the lower worlds, tried to rise up against them, initially by destroying the Gate of Judgment. Heaven's agents were sent against him, but Belldandy stopped them, protecting her mentor. More people are sent, and both are restrained. Celestin was sentenced to eternity in the Lunar Prison, whilst Belldandy's memories of the events were to be sealed away by Urd.

All is now ready for Celestin's new plan to change the world: if by destroying Yggdrasil he destroys the Earth, a new Earth will have to be created --- one free of suffering. The three Goddesses stand united against Celestin and Morgan Le Fay, trying to stop him. Meanwhile, in the Heavens, Peorth authorizes the use of Gungnir, to stop Celestin. She initiates the attack, but Belldandy protects him: they may be attacking Celestin, but it is Keiichi's body that would be destroyed! His eyes now opened to his erroneous ways, Celestin finally concedes, and releases Keiichi's body.

After this, Belldandy, Keiichi and Morgan Le Fay are transported to the Gate of Judgment. Having once lost her lover when trying to pass this test of the Gods, Morgan Le Fay warns Belldandy and Keiichi to not pass through it, but they move forward, with complete confidence they will pass it --- any who have no doubts of their love should pass the test, and not be separated. Upon stepping through the gate, the scenery is changed, and the two are still together. As they return to Earth, Morgan Le Fay tells them that she will stay and tell couples who come in the future the story of how they passed the test.

Belldandy and Keiichi return, but Yggdrasil is critically damaged. The Norns come together to weave one final spell, and rapidly restore The World Tree, before dematerializing it from Earth. Finally alone (though Urd and Skuld are actually spying from a tree not far away) Belldandy informs Keiichi that the virus deleted Yggdrasil's records, meaning that he is once again free to ask for any wish. Smiling softly, he replies, "That's easy," sparking their love for one another once more. Spying this from afar, Skuld wistfully wishes that she too will one day fall in love, just like Belldandy.

From Wikipedia