The mention of room 217 in 'The Shining' is one of those quiet, creeping details that builds up, you know? For me, the moment Danny sees the old lady in that bathtub—and the fact that it's specifically 217, not the 237 from the movie—creates this bedrock of terror that everything else in the Overlook rests on. It's not just a jump scare; it's the first concrete proof Danny has that the hotel isn't just spooky, it's actively malicious and lying in wait. His 'shining' gives him glimpses, but 217 is where the horror becomes undeniable and physical.
This event fundamentally shatters any illusion of safety for Danny, and by extension, for Jack and Wendy too, even if they don't believe him at first. It turns his fear from a vague unease into a specific, locatable threat. The trauma of that encounter makes him more withdrawn, more cautious, and it puts him directly at odds with the hotel's attempts to lure and use him. His entire arc becomes about resisting what he saw there, while the hotel uses that very fear to try and corrupt his father. The room is the catalyst; it's where the haunting stops being atmospheric and starts hunting.