"Inter-Dimension Detention", known as "Battle in a Different World! The Gem Beasts VS Harpie Lady" in the Japanese version, is the one hundred and twentieth episode of the Yu-Gi-Oh! GX anime. It first aired in Japan on January 24, 2007 and in the US on September 22, 2007.
After his narrow victory against Thelonious Viper, Jaden Yuki and co. arrive in an alternate dimension that's ruled by Duel Monster Spirits. The main building of Duel Academy was also transported there. The dorm buildings appear to have been left behind, however. Jesse Anderson ends up using "Crystal Beast Sapphire Pegasus" to fight off Harpie Lady. They determine that they can materialize their monsters using their cards, duel spirits are solid, and that everyone is able to see them. Although Jaden and Jesse are used to begin surrounded by spirits, they appear to scare all the others students and are therefore surprised when Sapphire Pegasus and Winged Kuriboh materialise. In addition, the basic rules of Duel Monsters, appear to apply, as Sapphire Pegasus with 1800 ATK was able to destroy Harpie Lady with 1300 ATK.
Adrian Gecko escaped from the collapsing lab with Devil Arm in tow. At the suggestion of Jesse Anderson, Dr. Crowler and Vice-Chancellor Bonaparte gather all the students in the gym and tally them. They determine that a total of 100 students were transported to the alternate dimension, and the rest (presumably in the dorm buildings), were not. Ms. Dorothy reveals that they have only one week's worth of food and water to ration. Along with the duel monsters problem, many of the students start to panic which leads to Jesse taking charge again and losing his temper with them. Once order is restored, Adrian arrives warning them someone is approaching Duel Academy.
Bastion Misawa arrives in the desert, and is attacked by Harpie Lady. He's saved by Adrian, while Jesse attempts to fight off the Harpie once more. He ends up facing "Harpie Lady Sisters" as the first Harpy was able to use magic similar to the "Elegant Egotist" card to multiply itself. Rather than destroying Sapphire Pegasus with an attack, the sisters are able to continually attack him until he collapses to the ground, wounded. It appears not all of the usual Duel Monsters rules apply though, as "Crystal Beast Topaz Tiger" is able to destroy one of the sisters without affecting the other two. Though Jesse defeats the sisters, they discover that the Bio-Bands are still active, and thus Jesse has some of his energy drained.
Bastion is taken to the infirmary, and he explains that after seeing Jaden's duel with Dr. Eisenstein, he became Eisenstein's assistant and helped him study duel theory. They developed a theory of their being twelve separate dimensions in which Duel Spirits reside. In an experiment, something went wrong and Bastion ended up being transported to the same desert dimension as Duel Academy.
Meanwhile Blair, Marcel and some other first-year students are hiding in a classroom, afraid of the duel spirits. Blair and Marcel head off to find the bathroom, unaware that they are being followed.
Back on Earth, Chancellor Sheppard has arrived back on Duel Academy's island and is shocked to simply see a crater where the main building should be. He is suddenly attacked by Echo. He is saved by two of Maximillion Pegasus' bodyguards, who have also just arrived via helicopter. Pegasus explains he received an urgent message from KaibaCorp.
Adrian gets back to the room where he had locked the Devil Arm in and discovers it's disappeared from it's capsule. Adrian determines that it found a more suitable host, someone with a great darkness in their heart than Adrian himself. The demon is shown, still taking the form of a glowing Pierce, searching. Yubel determines that Marcel Bonaparte would make a good host, and possesses him. Under her influence, he attacks and wounds Blair Flannigan, who is found in the hallway by Jaden later. Marcel is shown with the Devil Arm in place of one of his own.
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- As Jaden questions Blair about what happened to Marcel, her socks disappear.
The following cards appeared in this episode. Cards in italics debuted here.