Following the defeat of the Black Moon (the second story arc of the BSSM manga), things have been pretty quiet for the Sailor Senshi. That changes, however, when a new school for talented and gifted students suddenly becomes a center of mystery and intrigue. The school appears to not only have mysterious students like the handsome racecar driver Haruka and the beautiful Michiru, but also students who suddenly transform into black, ugly monsters. The team quickly realizes that there is something going on with the school and send Usagi, whose pen can give her any disguise, to investigate.

At the same time, Chibi Usa goes to a fair near the school to enjoy a fun day with her friends. While on the rollercoaster she losses her hat and leaves the park to go and find it. She recovers it in front of a building called “Tomoe Research Labs”, but she notices a girl collapsed on the ground, coughing terribly. She goes to help her, but the girl pushes her away, telling her that the place is restricted and that she is not allowed to be there. Usagi, who was just leaving the building, is surprised to find Chibi Usa there on her own, but they are soon engrossed in more pressing matters when one of the monsters appears.

Both Chibi Usa and Usagi transform, but when Chibi-Moon tries to protect Hotaru, the creature attacks her, biting her arm. Sailormoon uses her Moon Spiral Heart Attack and destroys the monster which turns out to be a small, white cat. Chibi-Moon checks to see if the young girl is unharmed, but the girl, who realizes that Chibi-Moon is Chibi Usa, replies, “You are the one who is hurt,” as she touches Chibi-Moon’s arm. Sailormoon notices to figures observing them. One appears to be dressed like Tuxedo Kamen and the other is a Sailor Senshi. As the two mysterious figures turn to leave Usagi’s friends and Mamoru appear. Suddenly Chibi-Moon exclaims, “Usagi! Look, my injury...” The mark on her arm has vanished. The dark haired girl wraps the wound with her handkerchief, tells her to disinfect it when she gets home, and explains that she has had this eerie power since she was a little girl. She also warns them that they should leave; otherwise they will be in trouble if the guards find them. Chibi-Moon asks for her name and the girl replies, “Tomoe Hotaru.”

As Hotaru enters the building, she collapses again in a fit of coughing. A tall woman with red hair rushes to her side to help, but Hotaru pushes her away violently. A man with a lab coat and a strange diamond piece over his eye, Professor Tomoe and Hotaru’s father, appears and admonishes her for treating the woman, Kaolinite, in such a rude manner, particularly since she has been so helpful since Hotaru’s mother died. Hotaru replies “"So she's ingratiated herself with you. She's your private secretary. Your assistant. I didn't know... She should know her place and not come into our home!" As she runs off to her room, Professor Tomoe remarks that he has spoiled her since her mother died.

Hotaru rests on her bed, still upset from the argument and in obvious pain. Her father appears to comfort her and see if she had taken her medicine. She replies that she did take her medicine but that it does not seem to be working. As he caresses her head in a fatherly gesture, he replies that in time the medicine will begin to work and offers her an amulet that belonged to her mother, telling her that it will help her sleep. As he leaves the room she thinks, “His hands were so cold..." The amulet seems to calm her pain and she reflects on how strong her attacks are becoming and wonders how long the sickness will last.