The "Door of Space and Time" is the door through Space-Time protected by the guardian of the underworld: Sailor Pluto.
Information[]
Sailor Pluto was assigned to protect the door by Queen Serenity during the Silver Millennium. She was tasked with protecting the kingdom from extra-dimensional threats or those attempting to reach it across time. She, under normal circumstances, would not allow anyone to pass through the door and will attack on sight. The door lies in its own realm but it is connected to doors within the Moon Castle and Crystal Palace.
The door can be reached though the use of a Space-Time Key which, though an incantation, will open a portal for the wearer to travel to Sailor Pluto and the door. The door is located in a dangerous realm which stretches far in all directions. Without a key, it can be easy to get lost.
Manga and Crystal[]
Long ago, Luna wandered the Moon Castle and stumbled upon this door. Queen Serenity soon found her and warned her to stay away as there was a guardian on the other side. Chibiusa also stumbled upon the door within the Crystal Palace one day and entered. She met Sailor Pluto and they became friends.
After the Black Moon Clan attacked, Chibiusa used Luna-P to help her steal a Space-Time Key from Pluto and she fled to the past. Eventually, she came back with Sailor Moon, Sailor Venus, Luna, Artemis and Mamoru but became separated from them. When they reached the door, Pluto refused entry and attacked them with Dead Scream. Chibiusa reappeared and after she spoke to Pluto she allowed everyone to advance to the 30th Century. They would pass through the space-time door to go back and forth from the future and present.
After Black Lady appeared and the situation became dire, Diana went through the Door to ask for Pluto's help. She promised to watch the door as Pluto was away. Pluto was able to save the others but died after she Stopped Time. The Senshi of the present passed through the door one last time to return to the past, but Chibiusa would continue to travel back and forth through time and came back as a trainee.
In the final battle with Pharaoh 90, the reborn Pluto used the Door of Space and Time as a part of Dark Dome Close to seal off the portal to the Tau Star System alongside Sailor Saturn.
When Chibiusa returns to the 30th Century in the stars arc, Sailor Pluto is back to her post inside the Door and Chibiusa decides to bring her flowers. She catches Neo-Queen Serenity speaking to Pluto about dangerous fluctuations in space-time before they notice her listening. The Queen allows Diana to head to the past, through the door, to investigate these problems in the past. After Pluto is incapacitated, she is brought inside the Crystal Palace with the also incapacitated Inner Senshi and King Endymion. Neo-Queen Serenity hands Sailor Chibi Moon and the newly awakened Sailor Quartet a Space-Time key to use to return to the past to help.
Anime[]
It was shown only in the second saga of the second season when Sailor Moon, Tuxedo Mask, and the Inner Sailor Guardians came with Chibiusa to reach the 30th century. Sailor Pluto suddenly appeared in front of the Door, intending to strike any trespassers with her Garnet Rod. After understanding their mission, she opened the Door in order for them to go to the future, Crystal Tokyo.