Growth
The Outer Senshi are basically MIA after the end of the Infinity/Super arc and reappear in the very end of the Dream/SuperS arc. As the Guardian Senshi battle with the Dead Moon Circus, Haruka, Michiru, Setsuna, and Hotaru are living a quiet and peaceful existence together and have been doing so for six months.
However, recent events cause the three "parents" worry - Hotaru is growing at an accelerated rate, her mental growth far surpassing her physical growth (which is also abnormal, since she looks like a young elementary school student at only six months!), and found that they were unable to transform when they felt an enemy's invasion onto Earth. They can do nothing, though, but wait. Then, one day, as Hotaru is going in for a bath, she is visited by the sleeping Saturn inside of her, reminding her of her past. As Hotaru remembers, she ages even more, settling at an age younger than her past incarnation and around the age of Chibiusa.
Hotaru appears to Haruka, Michiru, and Setsuna, who are shocked to see her growth. She gives them new Sailor crystals, allowing them to transform and warning them of the danger to their prince and princess. They set off to find the others, and after everyone gathers, the Senshi are able to save the Amazoness Quartet, use their powers to transform Sailor Moon to Eternal Sailor Moon (and get their third uniform upgrades) and defeat Nehellenia.
Afterwards, the Senshi return to their daily lives - Hotaru and Chibiusa are classmates and playmates. However, for the billionth (fifth) time, their lives are disrupted by the arrival of the Three Lights, a pop group, and the Sailor Animamates, who are revealed to be sent by a Sailor Galaxia to collect their Star Seeds. Star Seeds are crystals in everyone akin to Hostes, life sources, etc. It is basically a rehash of every previous season. At any rate, Galaxia is searching for the Star Seeds of Sailor Senshi.
In the anime, Galaxia reawakens Nehellenia in order to reawaken Sailor Saturn for her Star Seed. In the manga, no such intention is discussed and Nehellenia is just another villain, and Saturn is conveniently reawakened in time for Galaxia's search for Star Seeds.
Hotaru is upset to see Chibiusa return to the 30th century, but is faced with the problem of Galaxia's forces. The other Outer Senshi return to their castles to patrol the solar system, leaving Hotaru behind, but she later joins them, meeting her death at the hands of Galaxia while trying to aid Sailor Pluto. She is later "reborn" under Galaxia's control, turned against Sailor Moon. Sailor Moon, then, is able to defeat Galaxia and restore some degree of peace to the galaxy, allowing everyone to return to their "normal" lives, fighting evil by moonlight!
In summary, Sailor Saturn's story is most importantly found in the Infinity/Super arc.