How Do You Use the Mystery Key in Undertale?
Buy the Mystery Key from Bratty and Catty in the Trash Zone for 600G, then use it to unlock the house next to Napstablook's. It reveals the house of Mettaton's backstory!
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Mystery Key is sold by Bratty and Catty. Use it on the house near Napstablook's in Waterfall to unlock Mettaton's house.
You need to buy the Mystery Key from Bratty and Catty in Waterfall for 600G. Then, go to Napstablook's house and use it on the adjacent pink house. You'll access extra lore about Mettaton!
Just buy the Mystery Key from Bratty and Catty and use it to unlock the pink house in Waterfall adjacent to Napstablook's.
600G from Bratty and Catty -> Use on pink house near Napstablook鈥檚. Boom, Mettaton backstory unlocked!
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