While Maximillion J. 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 he managed to escape the castle.
Mokuba knew that Pegasus' plan involved defeating Yugi Mutou 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.
Dark Yugi, Katsuya Jonouchi and their friends saw the boy, whom Mokuba had stolen from, being restrained by Saruwatari 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 Saruwatari was unsympathetic. Jonouchi offered to catch the thief and asked that Saruwatari not remove the boy from the island until then. Jonouchi then added that he would keep half the Star Chips as a reward.
While searching for the thief, Dark Yugi and Jonouchi noticed the boat that eliminated participants leaving the island on 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 Dark Yugi and his friends. They recognized him as the thief, but did not immediately realize that he was Mokuba.
Dark Yugi accepted Mokuba's Duel request, despite knowing he was the thief. The two of them wagered their five Star Chips. As they lay down the chips, Dark Yugi 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, Dark Yugi's friends wondered why he accepted the Duel, when his opponent was clearly the thief.
1st turn: Mokuba
2nd turn: Dark Yugi
Dark Yugi Summoned "Winged Dragon, Guardian of the Fortress".
3rd turn: Mokuba
Mokuba Summoned "Crocodilus".
4th turn: Dark Yugi
Dark Yugi Summoned "Celtic Guardian".
"Celtic Guardian" attacked and destroyed "Crocodilus" (Mokuba: 1400 → 1100 Life Points).
Jonouchi thought that the thief was not very good and Anzu Mazaki agreed that he was no match for Dark Yugi. Dark Yugi 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 Dark Yugi caught him by the wrist, before he could escape. Dark Yugi 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.
Dark Yugi 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 Mokuba tried returning the Star Chips, Saruwatari appeared and warned them exchanging Star Chips outside of a Duel was against the rules and could be punished with disqualification. So, Dark Yugi challenged Saruwatari to a Duel for the Star Chips, however Saruwatari himself did not play games, so he arranged for the Ventriloquist of the Dead to face Dark Yugi.