Chibiusa doesn't do much in this volume since she goes home to the future to report on everything that's happened pretty early on. She does have a cute scene where she skips off to school holding Hotaru's hand as they gossip about Momoko and Kyusuke. How scandalous!
When Chibiusa leaves, she promises everyone that she'll be back soon and then runs off to go back to the future.
She doesn't show again until near the end of this volume when she shows up in Crystal Tokyo attending a small party to celebrate her return. Everyone is enjoying themselves until thunder rolls in. Hmm. Ominous.
Later that night she sits looking at some flowers and resolves to take them to Pluto in the morning and wonders about the sky. Things haven't been stable since she's gotten back to the future and having traveled through time more than anyone save Pluto one can assume she's developed some sensitivity to the time-stream's balance.
The next day she goes to take flowers to the time guardian's future incarnation (who had died, but was reincarnated, you follow) and happens to overhear Pluto and Serenity talking about some very bad stuff going down in the past that's making everything unstable and is possibly even changing history. Chibiusa runs off, intent on getting back to the past to help out.
But her mama has already made her turn in her time key permanently because she says her soldier training is complete and that Chibiusa must now stay in the future and study things there. Chibiusa wants to go back and help. She wants to fight, but she has no way to get to the past. Diana worries about her.