Volume 7 continues the Deathbuster Arc which started last book. Sailor Pluto makes her return after showing up briefly at the end of the last volume. We don't really get an explanation as to how she is back, other than I guess to help the rest of the outers.
The outers (Pluto, Uranus, and Neptune) explain that they are on the planet to stop the Deathbusters from destroying the planet and that the inner scouts can't help them. They each have a talisman which when the three are brought together will summon the scout from the "taboo" planet, Sailor Saturn, the Guardian of Silence who can destroy the world. They plan to eliminate her human host, who they identified as Chibi-Usa's new friend Hotaru before she awakens as a scout. The last time she woke up was at the end of the Silver Millennium and she heralded its downfall.
In the meantime, the talismans come together for a better purpose to create the Holy Grail which Usagi can use to transform into "Super Sailor Moon". That part of the storyline was adapted into the anime, although many other parts were either changed or ignored. I was rewatching a few episodes and found it interesting to see how differently everything happened.
The last chapter of this volume is a nice lead up to the final chapter(s). Hotaru's father is part of the Deathbusters and had used his daughter as a vessel for the alien Mistress 9. The Deathbusters had stolen Chibi-Usa's silver crystal which has awakened Mistress 9 who will use it to awaken Master Pharoh 90 who will theoretically try to destroy the planet.
I know basically what will happen, but unfortunately I never got to see the anime episodes because my cousin, who was supposed to tape them for me since I wasn't home, forgot until over halfway through the episode.