Volume Seven Part Two
Chibiusa doesn't do much in the first act of the Death Buster story other than annoy Usagi and get taken to the amusement park by Mamoru. She and Momoko and Kyusuke ride the roller coaster over the sea and she loses the hat she bought for Ikuko-mama. It goes into a restricted area and Chibiusa runs off to find it. When she does find it she runs into a little girl all doubled up and coughing on the steps of a research laboratory. She is worried and goes over to the girl but the girl tells her that she often has such fits and that Chibiusa should leave because it's a restricted area. Mamoru and Usagi run up to retrieve Chibiusa and Chibiusa and Usagi's brooches react. Something is definitely going down here, especially when a black mass coalesces into a monster.