In "Sailor Moon R Movie: Promise of the Rose," the narrative centers on a mysterious asteroid named Makaiju, which arrives on Earth and begins draining energy from humans. The asteroid is inhabited by a sentient tree and two alien siblings, Fiore and Kisenian, who seek to restore their dying homeworld. Fiore, a childhood friend of Mamoru Chiba (Tuxedo Mask), harbors deep resentment toward Usagi Tsukino (Sailor Moon) due to his jealousy over Mamoru's affection for her. Fiore's emotions are manipulated by the Makaiju tree, which amplifies his negative feelings and transforms him into a powerful antagonist.

The story unfolds as Fiore kidnaps Mamoru, intending to take him to a new world where they can live together without interference. Usagi and the Sailor Guardians—Sailor Mercury, Sailor Mars, Sailor Jupiter, and Sailor Venus—embark on a mission to rescue Mamoru and stop Fiore's destructive plans. Along the way, they confront Kisenian, Fiore's sister, who initially supports her brother's actions but later realizes the harm caused by their alliance with the Makaiju tree.

The climax occurs in space, where the Sailor Guardians face off against Fiore, who has merged with the Makaiju tree to become a monstrous entity. Usagi, driven by her love for Mamoru and her belief in Fiore's inherent goodness, uses the power of the Silver Crystal to purify the tree and free Fiore from its influence. The resolution sees Fiore and Kisenian departing Earth to find a new home, while Usagi and Mamoru reaffirm their bond.

The film explores themes of friendship, jealousy, and redemption, with a focus on the emotional conflicts between the characters. It also expands on the relationship dynamics within the Sailor Guardians and their unwavering commitment to protecting Earth and its inhabitants. The narrative is self-contained, offering a standalone adventure that complements the broader "Sailor Moon" series while maintaining its distinct identity.