Little Bear's Adventures of Sailor Moon R: The Movie is the first Little Bear/Sailor Moon crossover to be made by Hiatt Grey.
A young Mamoru Chiba hands a mysterious boy a rose before he disappears, vowing to bring Mamoru a flower. In the present, Mamoru meets up with Usagi Tsukino and the Sailor Guardians at the Jindai Botanical Garden. Usagi attempts to kiss Mamoru, but when he suspects the other girls of spying on him, he walks off outside alone.
The stranger appears from the garden's fountain and takes Mamoru's hands into his own, which makes Usagi uncomfortable. Usagi tries to break the man's grasp from Mamoru, but is knocked down. The man vows that no one will prevent him from keeping his promise before disappearing again. Mamoru tells Usagi that the stranger's name is Fiore (フィオレ Fiore). At Rei Hino's temple, the Sailor Guardians discuss an asteroid which has started to approach Earth and on which Luna and Artemis have discovered traces of vegetal life. The talk turns into rumors about Mamoru's and Fiore's possible relationship, while Usagi thinks about how Mamoru had told her that he had no family and was alone, and how she had promised him she would be his family from now on.
Fiore sends his flower-monster henchwoman, Glycina (グリシナ), to Tokyo to drain the population's life-energy, but the Sailor Guardians free them and destroy the monster. Fiore appears, revealing his responsibility for the attack, and uses a flower called a Xenian (キセニアン Kisenian) before severely injuring the Sailor Guardians. Mamoru attempts to talk Fiore out of fighting but the Xenian controls Fiore's mind. After Mamoru saves Usagi from certain death by intercepting his attack, Fiore takes Mamoru to an asteroid rapidly approaching Earth and begins to revive him in a crystal filled with liquid. While in the crystal, Mamoru remembers meeting Fiore after his parents died in a car accident. Mamoru had previously assumed that he had made up the boy as an imaginary friend. Fiore explains that he had to leave Mamoru because of the Earth's unsuitable atmosphere; Mamoru gave Fiore a rose before disappearing. Fiore searched the galaxy to find a flower for Mamoru, finding the Xenian in the process. Seeking revenge on the humans for his loneliness, Fiore returns to Earth.
Meanwhile, Luna and Artemis tell the Sailor Guardians that the Xenian can destroy planets using weak-hearted people. Ami Mizuno realizes that the energy from the asteroid matches the flower-monster's evil energy, deducing that Fiore has hidden there. The Sailor Guardians decide to rescue Mamoru. Despite her initial reluctance, the Sailors and Chibiusa convince Usagi to save Mamoru and confront Fiore.
After the Sailor Guardians fly to the asteroid, Fiore reveals his plans to scatter flower-seeds to drain humanity's energy on Earth. The Sailors Guardians then fight hundreds of flower-monsters, but they end up captured. When Fiore orders Usagi to surrender, she is unable to feel his loneliness; Fiore begins to drain her life-force. Mamoru escapes and saves Sailor Moon by throwing a rose at Fiore. The rose embedded in Fiore's chest blossoms, freeing him from the Xenian's control. The flowers on the asteroid disappear, but it continues to hurtle towards Earth. Usagi uses the Silver Crystal to transform into Princess Serenity to change the course of the asteroid. In an attempt to stop Usagi, Fiore soon realizes that when Usagi and Mamoru were children, she gave Mamoru the rose that was once given to him after Fiore had left. With Fiore and the Xenian destroyed by the Silver Crystal, Usagi, Mamoru and the Sailor Guardians combine their powers to divert the asteroid away from the Earth. The Silver Crystal is shattered and Usagi dies of exhaustion. Back on Earth, despite Luna and Artemis' concern over why the Sailor Guardians are taking too long, Chibiusa assures them that the girls are all right.
In the aftermath, the Guardians and Mamoru are devastated by Usagi's death, but Fiore reappears and thanks Mamoru. Using a nectar-filled flower with Fiore's life-energy, Mamoru wets his lips with the nectar and kisses Usagi, reviving her and repowering the Silver Crystal. Fiore, reduced to the form of a child again, returns to space to live in peace.