A shy, warm-hearted schoolgirl, Tomoe Hotaru was thrown into the middle of circumstances beyond her control. When she was young, she was horrifically injured in an accident while visiting her father's lab. An entity then appeared to the devastated Professor Tomoe, and he made a deal with it in exchange for his daughter's life. From then on, he was periodically possessed by a being known as Germatoid, and Hotaru by a being that was known as Mistress 9. Between that and possibly Sailor Saturn slumbering inside her, Hotaru became very sickly, frequently suffering from seizures and lapses of weakness. Her first appearance was in episode 111.
Like in the manga, Hotaru befriended and quickly grew close to Chibiusa, and soon Usagi and all the Inner Senshi, while she fell under the suspicions of Uranus, Neptune, and Pluto after she revealed to Haruka that Professor Tomoe is her father. They soon suspect that she was the Messiah of Silence (whom they connect with Sailor Saturn), and sought to kill her to prevent the awakening and thus destruction of the world, while Sailormoon, Chibimoon, and the Inner Senshi wanted to protect the innocent girl.
Eventually, Hotaru was possessed long enough to devour Chibiusa's Pure Heart Crystal, which finally fully awakened Mistress 9. Of course, Uranus and Neptune are shocked to not see Sailor Saturn, but a completely different being. Mistress 9 then deceived Sailormoon into giving her the Holy Grail, then put it into a large machine that blasted a large pillar of light to guide Master Pharoah 90 to Earth. She is injured by the blast, and Professor Tomoe, who was freed from the possession of Germatoid, took her into his arms. Hotaru's spirit, who Mistress 9 had thought died, then began to respond. Mistress 9 tried to subdue her again, but Hotaru's memories of her father and Chibiusa prove too powerful, and the mark of Saturn shattered the black star mark on her forehead. Mistress 9's body then disintrigrated.
After Sailor Saturn's sacrifice and the destruction of Pharoah 90, Super Sailormoon emerged from the ruins, cradling a baby Hotaru in her arms. She was returned to an injured Professor Tomoe, who had no memory of the previous events with the Death Busters. However, in episode 167, Setsuna took her from Tomoe, and she, Michiru, and Haruka raised her as their own. Remarkably, in the same episode, Hotaru emitted a powerful wave of energy that upgraded their Senshi forms into their Super forms, and grew rapidly into a small child. She then continued to grow rapidly, and in episode 168, she fully awakened as Super Sailor Saturn.
In episode 125, after Hotaru destroyed Mistress 9, Sailor Saturn's spirit appeared to Mamoru and the comatose Chibiusa, and returned her Pure Heart Crystal before she vanished. Saturn then appeared to Sailormoon, explaining how Pharoah 90 must be destroyed by a single blow from within, which Sailormoon cannot do because she can't even venture inside unless she was Super Sailormoon, and her role as the Senshi of Destruction. That she was to use her power that can destroy whole worlds, but would die if she did use that power. She bid Sailormoon farewell, before she threw herself into Pharoah 90.
Distraught that she can't transform into her Super form and stop Saturn from destroying herself, Sailormoon tried desperately and finally succeeded in transforming into Super Sailormoon one last time, and went in after Saturn. The power of the Ginzuishou then destroyed Pharoah 90, and Super Sailormoon emerged from the ruins, cradling a baby Hotaru in her arms.
Much later, in episode 168, the spirit of Sailor Saturn appeared to young Hotaru, to restore her memories and fully awaken her.
Super Sailor Saturn makes a surprising first appearance in episode 168, to deliver her message about her vision of the approaching darkness and danger to the Princess.
When Mamoru is kidnapped by Queen Nehelenia, Super Saturn and Super Chibimoon make it to her palace when all the other Senshi but Usagi are separated and captured in mirrors. As she watched Super Chibimoon continue to begin to flicker out of existance because Mamoru's possession changed the course of the future, she sees no choice but to bring ruin to everything to wipe out the Queen. Using her Silence Glaive Surprise attack, she nearly brings down her glaive in a wave of silence when Super Chibimoon stops her and reassures her to believe in their Princess. It was then with this distraction that Nehelenia imprisoned Saturn into a mirror with the others.
Much later in the season, long after Queen Nehelenia is stopped, Super Sailor Saturn appears again with Super Sailor Pluto to block a direct attack from Sailor Galaxia with her Silence Wall ability. Distraught over the loss of their Princess, the Starlights go on alone to attack Galaxia in her hideout in the Ginga TV station, while the rest of the Senshi ponder their next move. Haruka and Michiru insist on going alone to wipe out the invaders from outside the Solar System, and Setsuna and Hotaru later catch up.
After Sailor Chibi Chibi teleported Eternal Sailormoon and the Starlights outside of Galaxia's lair after having watched the four Inners lose their Star Seeds, and learning that Mamoru too lost his, Eternal Sailormoon awakens to Super Sailor Saturn standing over her. Super Sailor Uranus and Neptune realize that they have to put a stop to Galaxia, and head back to her lair alone, with Saturn and Pluto later following. When she realizes the sheer power of Galaxia, Super Sailor Saturn again attempts her trump card, bringing ruin to wipe her out in exchange for herself as well. However, Galaxia simply laughs and shocks her with the truth of why she awakened so quickly: she unsealed Nehelenia so she'd no longer have a "premature Star Seed." She then propositions the four Outer Senshi to give up their Star Seeds in exchange for unimaginable power. Shockingly, Super Sailor Uranus and Neptune agree, and proceed to carry out their first order: take Super Sailor Saturn and Pluto's Star Seeds. Their last words to Eternal Sailormoon before their deaths were reassurance that they believed in her, the Princess.
With the world saved and Galaxia healed to her former self in episode 200, all the Senshis' Star Seeds that were stolen were returned, and so were brought back to life. Super Sailor Saturn was no exception, and with that, her duty as a Sailor Senshi came to a close along with all the others.
Mistress 9, also known as the "Messiah of Silence", was the entity that possessed Hotaru ever since she was critically injured in her father's lab. Her purpose was to open a gateway for Master Pharoah 90 to Earth. The source of Hotaru's seizures, she dwelt in the girl, slowly gaining power as the Death Busters strived to awaken her.
She finally fully awakened in episode 123, when she stole and devoured Chibiusa's Pure Heart Crystal.
Mistress 9 then deceived Sailormoon into giving her the Holy Grail, then puts it into a large machine that blasted a large pillar of light to guide Master Pharoah 90 to Earth. She was injured by the blast, and Professor Tomoe, who was freed from the possession of Germatoid, took her into his arms. Hotaru's spirit, who Mistress 9 had thought died, then began to respond. Mistress 9 tried to subdue her again, but Hotaru's memories of her father and Chibiusa prove too powerful, and the mark of Saturn shattered the black star mark on her forehead. Mistress 9's body then disintrigrated.