The narrator and the Second Doctor are in an all-white, empty room with two doors. The narrator reaches for one, but the Doctor stops them. They reach for another, and the Doctor stops them again. The narrator looks around again and there is a third door. The Doctor says that one might not be safe and suggests the first door on the left. They both go through it to find another empty white room.
The Doctor says they're in the Multivarium, where anything and nothing is possible. He opens a door to find a vast white abyss. He turns around and the room is now a corridor. The Doctor thinks there are close to infinity doors, but the narrator thinks its just two hundred. The narrator suggests they don't choose a door. They sit in the corridor, but they can't tell for how long. The Doctor opens a door and flames leap out. The narrator opens a door, goes in, and falls asleep.
He is woken up by the Doctor in a different room. The Doctor explains the atmosphere in that room had nitrous oxide. They walk through a room with dust, leaving footprints that disappear when they leave. The Doctor says its impossible to enter or leave the Multivarium. The narrator doesn't remember how he got there. They walk on and on, not feeling hungry or tired, walking and waiting for years. The Doctor yearns for a chair. There are short rooms and cloud rooms and rain rooms. For a while, the Doctor marks doors with chalk. The rooms can change when the door is closed and opened again. The Doctor stops marking doors.
One room has too much gravity, and the Doctor and the narrators stand in the doorway until the pressure equalises. They try to predict if the next rooms will be fire, rain, or gravity. They get tired of opening doors.
The Doctor wonders if the narrator is actually the Doctor. He taps his foot against a door. He says he knows the lesson of the Multivarium, but the narrator doesn't care anymore. The tapping continues. A day later, the narrator tells him to stop tapping. The Doctor says he's trying to make a mark, to make the system a little less perfect. He stops tapping, and says the lesson of the Multivarium is that any action is pointless. The door is scuffed, and this gives them hope. The Doctor decides they need a system, and starts marking doors again, with a certain one for rain, fire, etc.
They come through an argon room to a room with four doors: gravity, fire, oxygen, and unmarked. The latter is locked. The Doctor says because the Multivarium contains all possible rooms, there must be a room where it is possible to leave. He lets the gravity leak into the room, and tells the narrator to open the oxygen door. He tells the narrator to go into the argon room, then opens the fire door. There is an explosion. The locked door is gone, with just blackness beyond.
- The Doctor drops his recorder into one of the rooms.
- The Doctor suggests a room could contain a particle of uranium and they wouldn't even know.
- The Doctor says "Kilroy was here" after leaving footsteps in the dust room.
- This story is told in the first person, and almost no information is revealed about the narrator, including their name, gender, or even species.
- This short story accompanied a chapter on the multiverse.
- The title is similar to that of the Doctor Who novel The Room With No Doors.
- Before he was in the Multivarium, the Doctor was fighting the Ice Warriors on the Moon with Jamie and Zoe. (TV: The Seeds of Death)