The Prep Room is a room in the Underground Shelter. It is a small room that connects the Lounge to the Decontamination Room. It is not labeled on any of the Ward Maps, but several characters refer to it as the Prep Room during the Decision Game.
The Prep Room has two doors - one leading to the Lounge and one leading to the Decontamination Room. It contains a vent that the teams believe lead to other wards, although it is later revealed that it went nowhere and only had a small area for Gab to rest in.
In D-END: 1, there is a protective suit located in the lockers in this room. It is unknown if the suit was located there in other timelines.
In one history, Phi devises a plan to use a bomb from Manufacturing to blow open a door in the Decontamination Room which leads to the elevator hall but is currently welded shut. Zero attempts to thwart her plan, however, by informing D-Team that the members of Q-Team are unconscious on the other side of the door, and that if the bomb detonates, they will die.
Diana immediately runs towards the Decontamination Room to disarm the bomb, but she is stopped by the acid showers in the room, which Zero has activated. Determined to get to the bomb, Diana searches the lockers and locates a protective suit.
In one timeline, Diana insists on wearing the suit, but fails to reach the bomb in time. She, the members of Q-Team, and possibly Sigma and Phi, die. In another timeline, Diana and Phi run away after Sigma convinces them that they don't have time to disarm the bomb. He stays behind and somehow locks the door to prevent them from getting back in. He then dons the suit and attempts to stop the bomb, although he also fails, losing his arms and eye in the process. The blast kills Eric instantly while Mira is mortally wounded. It is unknown what happened to Sean since he is a robot.