"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.
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
The Duel is shown from an unspecified turn. Maximillion Pegasus has 100 Life Points. His field contains "Thousand-Eyes Restrict" (who is equipped with Mai Valentine's "Harpie's Pet Dragon") (2000/2500) in Attack Position. Mai's Life Points are at 4000, and her field has "The Seal of Orichalcos" in play. She controls 6 face-up "Harpie Ladies" (1800/1400) in Attack Position (As "The Seal of Orichalcos" allows its controller to summon monsters in their Spell & Trap Card Zone). Neither player controls any face-down cards. Due to the effect of "Harpie's Pet Dragon", "Thousand-Eyes Restrict" gains 300 ATK and DEF for each "Harpie Lady" on the field ("Thousand-Eyes Restrict": 2000 → 3800/2500 → 4300).
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.
- This is the only episode from the original Yu-Gi-Oh! anime in which the Japanese and English dub share the same title other than episode 219 where the exclamation marks make little difference.
- The Seal of Orichalchos is shown to force Duels, even when the user's target hasn't consented.
- In the English Dub, Mai has a different voice actress, giving Mai a more cowgirl voice.
The following cards appeared in this episode. Cards in italics debuted here.
- This card was seen when Mai comes to the rescue and saves Yugi and his friends from the bikers.