When Seijuro and Natsumi miss school, Usagi goes to visit them at their apartment. She is attacked by the Doom Tree when she tries to peek into a secret room, but the aliens explain that it's just a botany experiment. Back at Rei's shrine, the girls ... (more)discover that Seijuro and Natsumi are really aliens by testing a twig caught in Usagi's hair. Later, Usagi runs into Mamoru outside of the apartment and tries to convince him not to visit. When he doesn't believe her, they end up going together. Natsum traps Usagi in the room with the Doom Tree and a cardian. When the cardian starts sucking Usagi's life energy, the Doom Tree starts growing. Eventually, the tree engulfs the entire building. The other Sailor Warriors arrive outside and try to find a way to get inside and save Usagi and Mamoru.