Key (Doors to Doom)

From the Super Mario Wiki, the Mario encyclopedia

The Key is an item in Doors to Doom. Mario and Luigi obtain it after finding it under a pile of coconuts in the first level of Donkey Kong Jr., Stage One. It is entirely green, and one side of the key is inscribed with "Positively Magic" while the other side is inscribed with, "Use once, then discard." Mario drops it in his pocket and the two continue onward. Later, at the end of Stage Four, they find Bowser in a cage. The padlock is labeled, "Insert green key and turn," and Bowser begs them to let him out. Meanwhile, Dr. Sporis von Fungenstein opens a doorway to peek on them and Donkey Kong Jr. charges after them. If they do not have the key, Mario and Luigi simply leave through the unlocked doorway; if they have the key, the doorway turns out to be locked and requires the same key as Bowser's padlock. As the key can only be used once, Mario and Luigi then decide where to use their key. If they free Bowser, he promptly says that he intends to leave them behind so that they get attacked by Donkey Kong Jr., but quickly slips on a banana peel, causing him to crash into Donkey Kong Jr., prompting a fight where the two characters eventually wrestle themselves off of the platform. In this case, Mario throws the now-useless key at the door in frustration, but for some reason this causes the door to open, revealing a path to Wart's lair. If they use the key on the doorway on the other hand to abandon Bowser, the door ends up leading to the beginning of Subcon instead.