Touhou Project Side Story 〜 星の記憶 (Hoshi no Kioku, literally "Memory of Stars") is a 50-minute fan-made animated video created by SOUND HOLIC and released at Comiket 73 in December 2007. Its plot involves Kaguya Houraisan, Eirin Yagokoro, Reisen Udongein Inaba and Sakuya Izayoi traveling back to the Moon to confront Kaguya's father, Luna Marius. The character animation is fairly rudimentary, but it makes extensive use of analog video effects.
The video was in production before Lunarian civilization was first canonically shown in Silent Sinner in Blue, which contradicts it. This movie's portrayal of the Lunarians, the Earth-Moon war, the Hourai Elixir and Kaguya and Eirin's parents should be considered non-canon.
As the story begins, Reisen receives a telepathic message from another moon rabbit, imploring her to help save the Moon. Shortly after, a Lunarian ship dispatched by Luna Marius, ruler of the Lunarian civilization, approaches the Earth and opens fire on Gensokyo, setting part of the bamboo forest on fire. The Lunarians demand that Kaguya be handed over and return to the Moon, which Eirin flatly refuses. As the Lunarians attack Eientei, Eirin and Reisen attempt to repel them but are stuck on the defensive, until Reimu and Marisa arrive suddenly and counterattack the Lunar ship, giving Tewi enough time to unleash her "Ancient Duper" Spell Card and destroy the ship.
Eirin demands to know how Reimu's seals were able to penetrate the Lunarian defenses, but the shrine maiden is at a loss to explain it, until Yukari appears and reveals that Gensokyo's borders are in a period of weakness as the powers of the Hakurei Border rises and falls over time. She then points out that the single-minded attention paid to the ship has left Kaguya vulnerable, and Reisen and Eirin arrive at Kaguya's chamber to find her space-freezing a multitude of knives thrown at her by Sakuya. Reisen interrupts the standoff and attempts to use her eyes against Sakuya, but Sakuya simply repels her gaze with lunar eyes of her own.
Sakuya subsequently reveals she was a spy sent by the Luna family to monitor Kaguya on Earth, and asks Kaguya to voluntarily return to the Moon, saying her power is needed. Kaguya, distrusting the motives of those who banished her to Earth, asks for an explanation, but Sakuya is unable to provide one; at this point Reisen once again hears the voice of another rabbit in her head. The rabbit, calling herself Kinako, explains that the "Mana" energy that sustains the Moon has become depleted by the war between the Lunarians and humans, causing it to lose its shine and weaken, and claims the Hourai Elixir could be used to make the Moon "immortal" (since Mana, like lifeforms, is a living thing). She also asks if "Rin-chan" is doing well, whom Reisen quickly surmises to refer to Eirin.
Kaguya and Eirin subsequently conclude their choices are limited; Marius will not stop attacking until they return. Leaving Tewi behind to protect Eientei, they depart for the Moon along with Sakuya using a teleportation corridor, as Sakuya laments having to leave Remilia's side. Upon their arrival, Kaguya agrees to assist in making the Elixir, but demands they be allowed to leave and return to Earth afterwards. Marius offers her the throne, but Kaguya rejects it in favor of her peaceful life in Gensokyo. Marius expresses surprise Kaguya has become so attached to Earth, but reluctantly agrees to honor her request.
Kaguya, Reisen and Eirin are welcomed into the laboratory where the Elixir is being prepared by Kinako--whom Reisen recognizes by her voice--to discover it already near completion. Kaguya finds an ordinary moon rabbit knowing how to manufacture the Elixir to be strange, as Eirin's research on it should have been destroyed centuries ago; however, Kinako reveals that she was once the assistant of Eirin's mother, Omoikane Yagokoro, and gleaned the knowledge of the Elixir from her. The only part of the Elixir she was unable to process successfully was the flower of Hourai, the final ingredient--a flower that blooms just once a millenium (and which bears a resemblance to Kaguya's own Jeweled Branch). Because of Kaguya's power of eternity, she can make the flower bloom at will, and proceeds to do so, creating three doses of the Hourai Elixir. Eirin is immediately suspicious of Kinako's knowledge; Omoikane was killed in a research accident while Eirin was a young girl, and Eirin doubts Kinako is old enough to have known Omoikane before she was born.
After Kinako shows Reisen a Mana crystal to demonstrate how the Moon is losing its light, she then opens a nearby door to reveal a large energy vortex, within which Sakuya can be seen trapped inside a similar crystal. Eirin quickly realizes they have been tricked by Kinako, but before anyone can do anything Kinako imprisons Kaguya, Eirin and Reisen in crystals as well. Marius appears to congratulate Kinako; Reisen demands they be allowed to return to Earth, and Marius says he will permit them, but only after he finishes the implementation of "Code E3". After Kinako sends one of the three Elixirs made from the flower into the Mana vortex, Marius orders her to bring him the remaining Elixir, leaving Kaguya and the others trapped in the crystal prisons.
Sakuya explains that Code E3 is a plan to "reset" the Earth by erasing the lifeforms from it. Earth was selected by the Lunarians long ago to be a planetary-scale laboratory where they could observe the process of evolution and propagation of life, and created humans based on their own genetic data to populate the Earth, as well as the monster races. Being aware they might not be able to control humanity's growth, they created a magical weapon called "Elysion" capable of destroying all life on Earth and returning it to its original state (not unlike Star Trek's Genesis Project). Eirin quickly deduces that the Hourai Elixir was actually needed for the Moon to provide sufficient Mana to use Elysion, and that Sakuya attempted to stop the plan and was caught. While they ponder the fate of Gensokyo, they are rescued from the crystals by Mokou, who came to the Moon by herself in an attempt to steal information about the Hourai Elixir.
As an Earth invasion fleet orbits the Moon, repelled by a protective dimensional barrier, the captain of the fleet, Sphere Sieben, receives a communication from Marius; he threatens the humans with destruction for turning against the Lunarians, but Sieben is unfazed by his ranting, implying to one of her subordinates that a surprise is in store for him. A sudden attack strikes Marius' headquarters and he turns to behold Kaguya and the others, escaped from their prison. Kaguya chastises her father for his hubris, and Sakuya and Reisen unleash their Spell Cards "Killing Doll" and "Invisible Full Moon" against him, but Marius laughs off their deadly assault and emerges unscathed. A stunned Kaguya realizes Marius himself must have taken the Hourai Elixir as well.
Marius announces he made a vow after losing "Selene" (presumably his wife, and Kaguya's mother) to obtain power greater than that of the gods; to this end, he drank the Hourai Elixir himself to become immortal. Kaguya is infuriated that she was betrayed by the father who committed the same 'crime' as she had and cast her to Earth while escaping punishment himself; Marius responds his bringing her back to the Moon was his "atonement" and his hands are clean. He hadn't expected her to prefer the Earth, but plans to render the question moot by destroying all life on Earth. Then he makes another revelation: consuming a second dose of the Elixir will further increase the power of the one who drinks it. Receiving another dose of the Elixir from Kinako, he consumes it, and then asks her what she desires in return for her service. Kinako's request catches him off guard: A desire for Marius' death.
As the others watch, stunned, Marius is overcome with convulsions and his body begins to break apart. Eirin is the only one who realizes what is happening: While the Hourai Elixir grants immortality to those who consume it, a second dose provides too much power for the body to handle. Like any drug, even the elixir of eternal life becomes a deadly poison when overdosed.
Marius demands to know the reason for Kinako's betrayal, and Kinako reveals the original Lunarian invasion of the Moon killed her family and friends and nearly her as well before she was saved by Omoikane. She had assisted Omoikane's research on the Elixir, not understanding at the time why Omoikane eventually stopped it, but now having realized that Omoikane was afraid of the Elixir being misused by Marius and his military. Eirin now deduces Kinako's true identity: a rabbit named Sakura who once assisted her mother--she had not recognized Kinako because her hair, which was once white, has now become black.
Kinako/Sakura goes on to reveal Marius murdered her and Omoikane because of Omoikane's refusal to continue the work on the Elixir, and their deaths were covered up as a research accident. Sakura, however, had managed to produce the Elixir and took a dose before Omoikane stopped the experiments, in hopes of living long enough to complete the research on her own. Thus, she survived Marius' attempt to kill her and plotted revenge against him, concealing her identity and gaining his trust. These revelations shock and horrify Eirin.
Marius, in agony as his body disintegrates, appeals to his daughter for help. Kaguya spurns him in disgust, telling him that compared to the courage of the humans who once challenged her Impossible Requests, he is a coward unworthy of her notice. Mokou, whose father once courted Kaguya, is stunned to learn that Kaguya actually held her failed suitors in such high regard after believing for centuries she had intended to humiliate them. As Marius collapses on the floor and dies, Sieben orders Earth's space fleet to land on the Moon and invade Marius' capital; apparently she had made a deal with Kinako/Sakura previously.
As Reisen questions Sakura's motivations for taking Marius' life, the black-haired rabbit responds by activating Elysion and then taunting the attackers from Earth. Sieben, realizing she too has been betrayed by Sakura, orders her fleet to stop their invasion and instead attack Elysion. Kaguya attempts to stop Elysion from firing, but to no avail. As the others decide the only way to stop Elysion is to destroy it in its orbit behind the Moon, Eirin spots Sakura slipping away. Eirin trails Sakura back to the Mana vortex and confronts her; she has now realized the claim that the Elixir affects Mana was a lie, and that adding the Elixir to Elysion will not give it enough power to fire.
Sakura responds that Mana is life; more specifically, it is fragments of the hearts of Moon rabbits. Rather than use Elysion against the Earth, her true plan was to use it to return the Moon to its natural state before the Lunarians arrived, one of pure Mana, using her own body. Before Eirin can stop her, Sakura hurls herself into the Mana vortex, taking the last of the Elixir with her, sacrificing herself to become Mana. As the others watch from space, a brilliant light radiates from the Moon and Elysion activates, changing its target from Earth to the Moon. It fires a laser at the Moon, wiping away the Mana and everything on the surface.
Reisen hears one final telepathic message from Sakura. Sakura admits she fell into the same cycle of hatred and retribution that took the lives of those close to her, and praises Reisen for having the courage to question the war and live peacefully. Eirin, meanwhile, comes face to face with Omoikane. Eirin reveals that Sakura showed her how to escape from immortality and join her mother, but Omoikane notes that Eirin still has no intention of dying, and implores her to return to the world of the living.
Eirin awakens to find herself on the surface of the Moon with Kaguya, Reisen, Mokou and Sakuya; Elysion's attack on the Moon has wiped away the Lunarian civilization, leaving a dead rock behind, and Eirin wonders if her meeting with her mother was merely a dream. Meanwhile, Sieben gives an order for the Earth fleet to pull back, bemoaning that simply wiping away one side of the war doesn't really settle anything. Reisen is struck with guilt at having been unable to save the Moon, but Eirin snaps her out of it, and they depart for Gensokyo.
Sakuya returns to the Scarlet Devil Mansion, where Remilia lectures her for having neglected the housework; meanwhile, at the shrine, Reimu asks Marisa why, if the Moon's civilization has disappeared, it now shines more brightly in the sky. Reisen, walking through the bamboo forest on her own, hears a bell ringing from the Moon...