What is the plot of Yu-Gi-Oh?

Context: Yu-Gi-Oh! tells the tale of Yugi Mutou, a timid young boy who loves all sorts of games, but is often bullied around. One day, he solves an ancient puzzle known as the Millennium Puzzle, causing his body to play host to a mysterious spirit with the personality of a gambler. From that moment onwards, whenever Yugi or one of his friends is threatened by those with darkness in their hearts, this other Yugi shows himself and challenges them to dangerous Shadow Games which reveal the true nature of someone's heart, the losers of these contests often being subjected to a dark punishment called a Penalty Game. Whether it be cards, dice, or role-playing board games, he will take on challenges from anyone, anywhere.

As the series progresses, Yugi and his friends learn that this person inside of his puzzle is actually the spirit of a nameless Pharaoh from Ancient Egyptian times, who had lost his memories. As Yugi and his companions attempt to help the Pharaoh regain his memories, they find themselves going through many trials as they wager their lives facing off against gamers that wield the mysterious Millennium Items and the dark power of the Shadow Games.

Answer: Yu-Gi-Oh! is about a boy named Yugi Mutou who plays a card game called Yu-Gi-Oh! He solves the Millennium Puzzle so his body now hosts the spirit of a gambler as well, and takes on various challenges against enemies. Most often, the challenge is the Yu-Gi-Oh! card game but sometimes it will be alternate games such as dice or board games. As the show moves forward, Yugi and his friends discover the spirit is actually an Ancient Egyptian Pharaoh with amnesia so they try to help him regain his memories by facing additional challenges from gamers with Millennium items.