When Hotaru Tomoe was very young, an accident happened in her father, Professer Souchi Tomoe's lab. She was injured and close to death. Her father was very sad, and worked for the Death Busters to bring Hotaru back to life. The Death Busters wanted to find the three Talismans which were hidden in three peoples' pure heart crystals. When the three Talismans are combined, they turn into the Holy Grail. The holy grail would wake the Messiah of Silence, who would be the one to call their master, Pharoah 90, and Pharaoh 90 would take over earth. Professer Tomoe created these daimons, who were monsters that would swallow the pure heart crystals of people. When Hotaru came back to life, her body was posessed by Mistress Nine...
From then on, Hotaru was a weak and lonely child. She had two personalities, Mistress Nine, and herself. Because of Mistress Nine, she had these seizures, which made her hurt her classmates. Due to this, she didn't have any friends. However, one day she meets ChibiUsa and they become very good friends.
The senshi soon find out that shes actaully the senshi of destruction, Sailor Saturn. The Outer senshi want to destroy her, because when Sailor Saturn awakes, the world would be destroyed. But because of ChibiUsa's friendship with Hotaru, the Inner senshi stop them from killing her.
Later, Kaolinite took ChibiUsa, and Hotaru swallowed ChibiUsa's pure heart crystal, and Mistress Nine took over Hotaru's body. Sailor Neptune and Uranus were about to kill her, but Sailor Moon wouldn't let them. Mistress Nine strangles Neptune and Uranus. Hotaru's father comes in, no longer evil. He goes over to Mistress Nine, and she says "Don't touch me, filthy man", and strangles him with her hair. Hotaru, was still alive in Mistress Nine's body, and kept trying to get out. Hotaru destroys Mistress Nine with her mind, and disappears. Later, she appears in Mamoru's apartment, to give ChibiUsa back her pure heart crystal.
Sailor Saturn then tells Sailor Moon that in order to destroy Pharaoh 90 and save the world from the Silence, she would have to go in and destroy everything. She tells Sailor Moon that when she destroys Pharaoh 90, she would die too. After the fight between Saturn and Pharaoh 90, Sailor Moon appears, with a baby in her arms. Hotaru has been reborned, and now has a new life.
She reappears in SailorStars, and is still a baby. Sailor Pluto picks Hotaru up from her father. She explains to Michiru and Haruka that they will soon need Hotaru's power. Later, during a fight, Hotaru powers up the Outer senshi. After the fight, Hotaru is much older then she was before. The outer senshi are suprised, because she was just a baby a few minutes ago. One night, Sailor Saturn comes to Hotaru. She gives Hotaru back all of her memories. Hotaru then appears in the room Michiru,Haruka, and Setsuna are in, and she is a lot older then she was before.
Hotaru becomes Sailor Saturn and tells Sailormoon about her true figure(eternal sailormoon).
After that Hotaru disappears again(ARG!) and reappears at the end of SailorStars. We find out that Galaxia was the one that revived her after she was reborned, because she wanted all the senshis' starseeds. Sailor Saturn dies with pluto, after their starseeds were taken out.
After the last battle, all the senshi are brought back to life. Hotaru, Setsuna, Haruka, and Michiru are in front of a car. They all see a shooting star and make a wish.