In the beginning of Sailor Moon R: The Movie, Usagi is shown daydreaming about Mamoru until Luna interrupts her and reminds her of their duty as Sailor Guardians. They go to visit a botanical garden, where they meet Mamoru's childhood friend Fiore. Fiore is an alien who has been living in space and has returned to Earth to give Mamoru a special flower that represents their friendship.
However, Fiore becomes possessive of Mamoru and dislikes Usagi and the rest of the planet's inhabitants. He becomes controlled by an evil flower called the Kisenian Flower, which can manipulate people's emotions and drain their life force. Fiore is sent on a rampage to collect human energy to feed the flower. The Sailor Guardians must work together to stop the Kisenian Flower before it becomes too powerful and destroys the planet.
Throughout the movie, Usagi struggles to reconcile her jealousy of Fiore's relationship with Mamoru and her duty as Sailor Moon to protect the world. She forms a strong bond with Fiore's loyal robotic assistant, Glycina, who helps the Sailor Guardians in battle. In the end, Usagi confronts Fiore and manages to break through the Kisenian Flower's control by reminding him of their childhood friendship and how much Mamoru still cares for him. Fiore sacrifices himself to destroy the Kisenian Flower, and Mamoru and Usagi embrace in a poignant scene.