The Spaceship is a location only accessible after the player has found all five Jellyfish. Once found, a red symbol in the regular ending hallways can be activated, which causes a larger version of the same symbol to appear on the roof. If interacted with, Madotsuki will be teleported to the Spaceship.
The Spaceship is essential for obtaining the Secret Ending.
The Spaceship consists of three rooms - the entry room, the main room, and the bedroom. Seccom Masada-sensei can be found in the main room, playing the piano. If Madotsuki completes the song he's playing, the bedroom door will open.