In his youth, Mamoru hands a mysterious boy a rose before he disappears, vowing to bring Mamoru a flower. In the present day, Mamoru meets up with Usagi Tsukino and the Guardian Senshi at the Jindai Botanical Garden. Usagi attempts to kiss Mamoru, but when he suspects the other senshi of spying on him, he walks off outside alone.
The stranger appears from the garden’s fountain. 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. At Rei’s temple, the senshi 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 gossip 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 a flower-monster, Glycina, to Tokyo in order to drain the population’s life-energy, but the Guardian Senshi free them and destroy the monster. Fiore appears, revealing his responsibility for the attack, and uses a Xenian flower before severely injuring the senshi. Tuxedo Mask saves Sailor Moon from death and tries to talk Fiore out of fighting. However, the Xenian flower controls Fiore’s mind and has him attack Sailor Moon. After Tuxedo Mask saves her from being killed, 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 flower in the process. Seeking revenge on the humans for his loneliness, Fiore returns to Earth.
Meanwhile, Luna and Artemis tell the senshi that the Xenian flower can destroy planets using weak-hearted people. Sailor Mercury realizes that the energy from the asteroid matches the flower-monster’s evil energy, deducing that Fiore has hidden there. The Senshi decide to rescue Mamoru; Sailor Moon is reluctant to do so, but the Senshi and Chibiusa convince her to save Mamoru and confront Fiore.
After the Senshi fly to the asteroid, Fiore reveals that he will scatter flower-seeds to drain humanity’s energy on Earth. The Senshi then fight hundreds of flower-monsters, but the Guardian Senshi are captured. After Fiore orders Sailor Moon to surrender, Sailor Moon 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 in order to change the course of the asteroid. Fiore tries to stop Usagi’s transformation, but he soon realizes that when Usagi and Mamoru were children, she had given Mamoru the rose that was once given to him after Fiore had left. The immense power and energy from the Silver Crystal obliterate Fiore and the Xenian. Mamoru and the Guardian Senshi lend Princess Serenity their powers and abilities to sustain the Silver Crystal as the asteroid descends towards Earth. The Silver Crystal shatters, the asteroid disintegrates and Usagi dies. On Earth, despite Luna and Artemis’ concern over why the senshi are taking too long, Chibiusa assures them that the senshi are all right.
Meanwhile, on the disintegrated asteroid, the girls and Mamoru cry over Usagi, 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.