What pathway do I have to take to find Ending #1 in 999?
You must first finish the ending where you open the safe (which it sounds like you have).
Then go through doors 4, 7, and 1, making all the right choices to let people blab at you.
The "right" choices are
- Take the 4 leaf clover from santa
- Let June talk about the titanic (room 4) and Ice-9 experiment (in the freezer)
- Give it to clover in room 7 and talk to 7 about the Ice-9 experiment
- Say "Patch it up" to clover's question in door 7
- Yell at Ace to get out of room 1 when prompted
- Look at monitor after ZERO is spelled out on screen
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