Chibimoon is very impressed with this new Sailor Pluto who calls her first "Small Lady" and then corrects herself to "Sailor Chibimoon." She explains that Neo Queen Serenity lent power so that she could be reincarnated in time for her new duty. Chibimoon is elated to know that Pluto is alive again, but Pluto doesn't seem to have much time to do much of anything other than concentrate on her duty. Chibimoon's only friend is now too busy for her. She will have to look elsewhere, although Pluto will always keep a helpful and guiding eye on her.
After the battle is wrapped up, Chibiusa goes home with Mamoru who wants to do some more research on Tomoe. Luna tells Chibiusa that she must keep an eye out when she visits Hotaru. It is her duty as a Sailor Soldier to fight evil and nothing can get in the way of that. Chibiusa is very impressed by actually having a duty (any duty, really) as a Sailor Soldier.
The next time she runs into Hotaru is when Hotaru invites her (and her friends) to the movies. She's ecstatic, and Luna and Diana both tell her that she's doing a good job as a Sailor Soldier. As she's preparing to leave, Kenji-papa admires her Grail.
Chibiusa, Momo, and Kyusuke wait a good solid hour for Hotaru at the movies but she never comes. Chibiusa becomes very worried and sends Momo and Kyusuke ahead into the movie while she runs to the research lab to check on Hotaru. The door is unlocked and no one answers so Madamoiselle Queen of Manners and Others' Wellbeing lets herself in. The inside of the lab is very cold.
Hotaru is late because her papa was busy installing new wires and transistors in her because she is a CYBORG. Gasp! Chibiusa accidentally sees this through a window and then loses it and runs off, because well, that's a little weird. Hotaru is very distressed that Chibiusa has run away and thinks that now she's lost the only friend she's ever had. She throws the crystal that her father gave her away, out the window and her head feels as if it will burst, because, get this, something is TRYING TO GET OUT. Creepy.
After Chibiusa calms down a bit she feels guilty. She knows that Hotaru was seriously injured in an accident long ago and that she hadn't wanted to show the scars to Chibiusa. She worries that she's hurt Hotaru, but before she can go back and apologize weird hail starts falling from the sky and she gets weak and collapses.
Fortunately Mamoru and Usagi find her lying on the ground and carry her back to the control center. There the inner senshi are preparing to leave for a fight that's brewing in the Mugen Center. Chibiusa begs to go, but Usgai says it will be too dangerous and leaves her with Mamoru in the Crown.
Of course, Tuxedo Kamen and Sailor Chibimoon aren't about to stay out of the fight and arrive just in time for Chibimoon to attempt to break up the sailor senshi who are feuding due to witch craft. Chibimoon, Sailor Moon, and Tuxedo Kamen all combine forces and join hands to produce the Holy Grail.
Sailor Moon uses Crisis Power to become Super Sailor Moon and Chibimoon remembers that King Endymion told her before she left that someday she would see Sailor Moon reach a much higher power level: Super Sailor Moon. Chibimoon knows what's going on even if no one else does.
Shortly thereafter Pluto, Uranus, and Neptune explain that they are trying to thwart the rebirth of the Destroyer, Sailor Saturn. Neptune shows them Sailor Saturn's reflection in the mirror and Chibiusa and the others are distressed to see that it is Hotaru. Uranus tells Chibimoon that they must destroy Hotaru before Saturn can awaken. She says that Hotaru's body is all worn out anyway, half machine and broken. The only way she can return to health is to become Sailor Saturn and they cannot allow the Destroyer to waken and end the world.
Chibimoon can't stand to hear the talk of killing Hotaru whom she loves like a best friend so she runs off intent on finding Hotaru and saving her. She climbs the fence at the lab and lets herself in again. The outer senshi are watching from the trees, but they can do nothing as Chibimoon approaches Hotaru who has collapsed in the doorway. An evil hand grasps at Chibimoon and yanks her brooch away and Chibiusa reverts and collapses on the ground. Usagi and the others arrive just in time to see this. Mistress 9 has awakened and Chibiusa is cold and is no longer breathing. Dude, this really sucks.
Chibiusa is taken to the hospital where a doctor confirms that she has gone into cardiac arrest because the ginzuishou and her soul have been stolen. The prognosis isn't good because she will soon die or turn into a daimon. Her body is already stiffening. The only way that anyone can think of to save her is to destroy Mistress 9 and retrieve the ginzuishou, and this also means killing Hotaru. Usagi is not hot about this at all, but she must save Chibiusa . . .
Mamoru can keep Chibiusa's body from decaying while Usagi and the others fight by expending his own power to keep her heart beating and her blood flowing. He agrees to stay behind with Chibiusa while Usagi goes to fight.
While fighting, Sailor Moon is accosted by hallucinations of all her comrades, including Chibimoon who jumps on her back like an evil little monkey and tries to strangle her. Sailor Moon is too strong to be fooled by this and breaks out of it. She meets up with the outer senshi and they go to face Mistress 9 after defeating Kaolinite.
Unfortunately for them, Mistress 9 has decided to go nuts and change the Mugen Zone into a nightmare using the power of Chibiusa's ginzuishou. Chibiusa begins to glow when her power is again activated. Uh-oh, this can't be good.
Chibiusa's spirit is alone and scared, and then Hotaru begins to fight Mistress 9's control and safeguards Chibiusa's soul. Chibiusa thinks Hotaru's arms are very comforting, like her mother, yet cool and deep. Hotaru vows to protect Chibiusa at all costs. Through Hotaru's protection the warmth comes back into Chibiusa's body and her blood starts to flow again without outside help.
Later, after protecting the souls of the inner senshi Hotaru's spirit manages to burst outside of her body. The souls return to the inner senshi and she personally takes Chibiusa's soul back to her. Chibusa wakes up and she and Hotaru agree that it was fate that they met. They clasp hands and then far away in the Delta District Mistress 9 becomes a monster. Hotaru screams in pain, and then says goodbye to Chibiusa before fading away.
Chibiusa is sure that she's died and starts to cry because this is the second time she's lost her best friend to DEATH. Mamoru consoles her and then together they resolve to go to the fight and combine forces to help Sailor Moon.