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The Master Key is a key found in Wario: Master of Disguise. Wario can find it in a purple treasure chest in a room on the upper-right corner of the S.S. Caviar during Episode 2. The room can be accessed by riding the elevator to the fourth floor and going to the right. The key opens two locked doors on the ship.
After Wario obtains the key, the number 2 appears next to the key's icon on the top screen. After unlocking one of the doors, the number changes to a 1, implying that the Master Key is actually two keys. However, when obtained, Wario refers to the key as a singular entity. It shares this trait with the Yellow Key and Red Key.