Door 1 is one of three choices in the third set of numbered doors the Nonary Game participants find themselves faced with. The other two are Door 2 and Door 6. Unlike the previous sets of numbered doors, these doors are not on the same deck as one another, let alone in the same room.
The only way to get to Door 1 is by using the Earth key on the corresponding door by the main staircase. It leads to a short hallway with the numbered door at the end.
After going through all of the doors in the large hospital room and discovering Snake's fate, the remaining players decide to vote in order to decide who would go through which door. They write their choices on pieces of paper from Junpei's notebook to make sure no one could change their vote. The results were:
Unknown to the others, Junpei has three papers hidden, one in the pile and one in each hand. This way, he can change his vote after reading everyone else's choice and choose which door he wants. He put the Door 6 paper into the pile and put a symbol on the paper to make sure he drew last. Santa becomes suspicious of Junpei's choice after Junpei pulls the Door 1 paper, but ultimately finds no proof that Junpei pulled anything weird and left Junpei's plan unexposed.
Junpei's choice of Door 1 leads him to go through it with Ace and Clover.
1+4+5 = 10 = 1+0 = 1
Since the other two groups won't work without compromise, Seven and Lotus decide to go through Door 6 with June and Santa.
3+6+7+8 = 24 = 2+4 = 6
With the issue settled, the groups head off their respective doors.
Behind Door 1
The DEAD of Door 1 is right next to it on the other side. Behind the door is the chart room and the wheelhouse.