"An Unexpected Enemy" is the 151st episode of the second series anime. It first aired in Japan on April 15, 2003, and aired in the United States on October 23, 2004.
Kaiba, still reeling at the battle he's fought, decides to go see Pegasus despite an urgent mail from his company requesting his presence there. His plane is refueled in mid-flight.
The driver of the limo which was going to drive Yugi Muto and his friends to Industrial Illusions has ditched them next to an abandoned oil stand. A group of bikers hired by Doma come and try to attack Yugi & co. However, Mai comes to the rescue with her bike and single-handily defeats all of them. The others don't realize in that moment, but she sports a Chaos Duel Disk, meaning that she's in league with Doma. Rafael and Valon are observing from a close distance. Valon looks rather annoyed with Mai's actions. The group flee in the limo, which breaks down, so they hitch-hike a ride towards the I2 HQ.
Weevil Underwood and Rex Raptor wander aimlessly around the downtown streets of San Franciso. Just then, a man robs Weevil's wallet and runs off, only to be stopped by a dice hitting his face: Duke Devlin is here. The man drops the stolen wallet and runs off. Duke offers them a ride on his car and they make up a convincing tale to make it look they're worried about Yugi & co. because of the God Cards theft. They also head towards the I2 building.
Yugi, Joey, and the gang arrive at Industrial Illusions, and the doors lock behind them. The gang is confronted by Mai Valentine. She reveals that she has joined Doma because of her loneliness and overwhelming drive to become the best Duelist. She reflects on her first assignment -- challenging Pegasus. Even the creator of Duel Monsters was not able to beat her, and although his soul is taken, she wants to prove her power elsewhere. Joey wants to free her from this influence, but he's the one that Mai wants to Duel first. He refuses, but she starts anyway and activates "The Seal of Orichalcos", trapping him and determining that one of them will lose their soul. Even as she ends her first Turn, he still does not want to duel her, and he is at the brink of despair. Nothing he says is getting through to her, and fighting back would only mean that her soul could be captured.
Featured Duel #1: Mai Valentine vs. Maximillion Pegasus
Featured Duel #2: Mai Valentine vs. Joey Wheeler - Part 1
Turn 1: Mai
Mai draws "The Seal of Orichalcos" and subsequently activates it. From now on, all of Mai's Monsters will receive a +500 ATK boost and the loser of this Duel will lose his/her soul. Mai then Normal Summons "Harpie Lady" (1300 → 1800/1400) in Attack Position.