Sugoroku Mutou believed the hieroglyphics on the box read "The one who solves me shall gain the powers and knowledge of darkness." Yugi believed they said something along the lines of "Whoever solves this puzzle shall be granted one wish." and wished for friends. He brought the puzzle with him everywhere, usually keeping it inside the box, and spent eight years trying to solve it.
One day at school, Katsuya Jonouchi and Hiroto Honda overheard Yugi talking about the box. They took the box and teasing him over it, with Jonouchi taking a piece, without him noticing. Yugi got the box back when Anzu Mazaki intervened.
When Yugi solved the puzzle, he kept the box to store prized possessions such as cards. Time Wizard was one special card kept here later given to Joey.
The design of the Millennium Puzzle box became a popular motif for Yu-Gi-Oh! merchandise. The packaging for Yugi's Legendary Decks was directly based off of it.
- Yu-Gi-Oh! Duel 1: "The Puzzle of the Gods"