Chibi-Usa tells Sailor Venus, Sailor Moon, and Tuxedo Kamen that she is from the future. Her home is Crystal Tokyo, and it has been under siege from the Black Moon. She came to the past to save her mother, but she's not sure how to do it. They ask if Chibi-Usa can take them back to the future with her, but she says that she hasn't finished her job yet. Sailor Moon suggests that they go home and rest, but Chibi-Usa suggests "Mamo-chan's house."
At Mamoru's apartment, Chibi-Usa is dressed in one of his button-down shirts. He tucks her into his bed and says that she can sleep there. He asks if she came to the past by herself, but Chibi-Usa said of course not: she came with Luna-P. Mamoru feels bad that this little girl went through all this with only Luna-P.
After Mamoru leaves Chibi-Usa, Usagi asks if she can stay a little longer. A little apprehensively, Mamoru says that she can. Usagi wanders around nervously. She picks up a calendar and says that it's almost Mamoru's birthday: what does he want? Mamoru stares at her in disbelief. "What's come over you?"
Finally, Usagi says what has been plaguing her. "You're always thinking about Chibi-Usa. Is she pretty? Do you like her now?"
Shocked, Mamoru downs a cup of coffee. "But, she's just a schoolgirl!"
Usagi yells that being young has nothing to do with it, but she realizes what she's saying. Embarrassed, she apologizes and turns away from him. To herself, she admits that she's been jealous.
Mamoru puts his arms around her and hugs her tightly. "I don't want to lose you, Mamo-chan," she says.
"I don't want to lose you either, Usako. I want to be with you always. I want to protect you." He lays her on the floor and lies on top of her. They kiss deeply, and Usagi wraps her arms around him....
Minako lets the cats stay with her for the night. Artemis tries to talk to Luna about the possibility of a time warp, but Luna falls fast asleep.
Luna has a flashback dream. Queen Selenity stops Luna from entering a door surrounded by fog. She says that the door is defended by the "solitary guardian." Anyone who passes through that door will be able to travel through time, a forbidden activity. Luna wakes up and remembers the crystal hanging from Chibi-Usa's neck.
The next morning, Chibi-Usa wakes up. Usagi (now wearing Mamoru's shirt over her dress) and Mamoru enter the room. "I'm going to the future," Chibi-Usa says. "Usagi, Mamo-chan. Come with me."
That night, outside the Game Center, Sailor Moon, Tuxedo Kamen, Sailor Venus, and the cats watch as Chibi-Usa raises the key into the air. She explains that the key is the Space-Time Key. She remembers a dark-haired Senshi telling her that time travel is forbidden. Nevertheless, she calls into the sky: "Time Guardian! Tear apart the sky, and open the space-time door to me! I call the true name of the almighty god of time, the time guardian's father! Chronos! Reveal to me the path of light!"
The sky opens up in a swirl of light. Chibi-Usa starts to float upwards. Sailor Moon grabs her hand, but she loses her grip, and Chibi-Usa disappears.
At the Black Moon headquarters, Safir says that someone has opened the time warp. Everyone wonders if it was the Rabbit, but the energy is too high for it to be just one person. The Wiseman appears and tells Diamaund to destroy the Ginzuishou, the Rabbit, and the castle. Coming to the prince's aid, Esmeraude calls forth her minions, Kiral and Akiral, the Bull Brothers. "Code: 005. Operation: Remake. We will use the Evil Black Crystal to create the maximum chemical reaction and take their beautiful leader."
Sailor Moon, Tuxedo Kamen, Sailor Venus, and the cats are stranded in the dark. A hole opens up in the darkness, but a voice in Tuxedo Kamen's head tells him not to go in there. Sailor Moon's brooch begins to glow as a door appears from within the mist.
A woman blocks their path to the door. She has dark skin and hair. Her sailor fuku is black, and she carries a staff that looks like a large key. She tells them that she is the guardian of the space-time door, Sailor Pluto. Her mission is to eliminate those who try to enter the door. She tips her staff at Sailor Moon. "Dead Scream!"
"Wait!" Chibi-Usa runs up to Sailor Pluto. She jumps into her arms and hugs her. She explains that she brought the others to the door because she thought that Sailor Moon's Ginzuishou would be useful. Sailor Pluto forgives her; she's just thankful that "Small Lady" is safe. She kneels to Chibi-Usa and calls her "Princess."
Sailor Pluto steps aside and lets them enter the door. Outside, the city is made entirely of crystal. Crystal Tokyo is very quiet, unlike a normal city. Sailor Venus looks around and gasps: there are decaying corpses lying on the ground.
Tuxedo Kamen sees a giant black monolith standing in a clearing. Chibi-Usa hides behind him. She says that there was a huge explosion that destroyed most of Crystal Tokyo. Everyone in the city was killed. She starts to take them to the Crystal Palace to see her mother.
Before they can get there, Kiral and Akiral attack. Sailor Moon tries to use Moon Princess Halation, but her rod doesn't work. The Bull Brothers create a crystal wall and trap the group. Tuxedo Kamen hears the voice again; this time it tells him that the wall is not real crystal. He smashes the wall with his arm, and Sailor Venus uses her Love Me Chain. The Bull Brothers are destroyed by Tuxedo Kamen's Tuxedo La Smoking Bomber.
They find themselves standing in front of the Crystal Palace. They enter. In the center of one of the rooms, there is a large platform upon which rests a mass of crystal. Inside the crystal, a woman lies unconscious; she wears a white dress and has her hair done up in two odango. Chibi-Usa tells them that the woman is Neo-Queen Selenity.
A gray kitten jumps into Chibi-Usa's arms. "Diana!" The kitten jumps to the floor and run to a curtain. A man steps out.
The man is wearing a lavender tuxedo and cape. His dark hair is short in the back and long in the front, just like Mamoru's. A white mask covers his eyes. "Tuxedo Kamen!?"