While Maximillion Pegasus tried taking over KaibaCorp, Mokuba swallowed an important key, so Pegasus had him kidnapped until the key could be retrieved. Mokuba was brought to Pegasus Castle on the Duelist Kingdom island. However Mokuba escaped the castle.
Mokuba knew that Pegasus' plan involved defeating Yugi Muto in the finals of the Duelist Kingdom tournament, so Mokuba tried to eliminate Yugi from the tournament before that could happen. He stole a Deck and five Star Chips form one of the participants and planned to use them to defeat Yugi.
Yami Yugi, Joey Wheeler and their friends saw the boy, whom Mokuba had stolen from, being restrained by Kemo and told he was to be sent off the island for losing all his Star Chips. The boy explained that the Star Chips had been stolen, but Kemo was unsympathetic. Joey offered to catch the thief and asked that Kemo not remove the boy from the island until then. Joey then added that he would keep half the Star Chips as a reward.
While searching for the thief, Yami and Joey saw the boat eliminated participants leaving the island and among them was the boy whose Star Chips had been stolen, meaning they would not be able to return the Star Chips to him. Mokuba masked with a bandanna appeared before Yami Yugi and his friends. They recognized the person before them as the thief, but did not immediately recognize that he was Mokuba.
Yami accepted Mokuba's Duel request, despite knowing he was the thief. The two of them wagered their five Star Chips. As they lay down their Star Chips, Yami noticed that Mokuba did not have a Duel Glove, meaning he was not invited to the tournament. He wondered if the thief stole the Star Chips just to challenge him and started to become suspicious of something. Meanwhile, Yami's friends wondered why he accepted the Duel, when his opponent was clearly the thief.
1st turn: Mokuba
2nd turn: Yami
Yami Summoned "Winged Dragon, Guardian of the Fortress".
- Mokuba's turn
Mokuba Summoned "Krokodilus".
- Yami's turn
Yami attacked "Krokodilus" with "Winged Dragon, Guradian of the Fortress".
Joey thought that the thief was not very good and Téa agreed that he was no match for Yami Yugi. Yami advised Mokuba to give up, as the game is not so simple that it can be won with stolen cards. Mokuba knew he did not have a chance of winning, so tried to take Yugi's Star Chips instead.
The Duel was not finished.
Mokuba made a grab for the Star Chips, only managing to grab seven of the ten, but Yami caught him by the wrist, before he could escape. Yami then removed Mokuba's cap and bandanna, revealing his identity.
When questioned about why he tried to steal Yugi's Star Chips, Mokuba told them that it was not for revenge. He told them about Pegasus' plan to take over KaibaCorp, which required Pegasus to defeat Yugi in an official Duel. Mokuba had thought if he could make Yugi lose all his Star Chips, he would be disqualified before Pegasus could Duel him.
Yami and his friends managed to convince Mokuba that they would help him and somehow find a way to beat Pegasus, whom Mokuba had heard was unbeatable. However, when Yugi and co. tried to find the boy, it was too late, as the boat with the losing duelists, the boy and Weevil Underwood included, already left. When Mokuba tried returning the Star Chips he stole from the boy to Kemo, he threw them into the water and warned them exchanging Star Chips outside of a Duel was against the rules and could be punished with disqualification. So, Yami challenged Kemo to a Duel for the Star Chips Mokuba stole from Yami, however Kemo himself did not play games, so he arranged Ghost Kaiba, as an opponent for Yami.
- Yu-Gi-Oh! Duelist Duel 14: "The Thief"
- Yu-Gi-Oh! Duelist Duel 15: "Messenger From Hell"
- Yu-Gi-Oh! episode 8: "Everything's Relative"