After a harmless misunderstanding, Shoma now finds himself in a duel against the (self proclaimed) servant of darkness - Dark Kuroda.
In apartment 201, Ageha tries to snap Shoma out of his state. However, Shoma, who is trying to build a deck, is unresponsive, as he has another fantasy. He goes back to the Tenyi world, where he encounters "Tenyi's Dragon Ogre Deity". The latter recommends Shoma to include 3 Adhara and 3 Vishuda monsters, which Shoma has obtained. The berserker is glad, as Shoma can use those monsters to Synchro Summon it. The monk and shaman of the Tenyi appear, too, confusing Shoma, since he did intend to put them both in the Deck. The monk wags his finger, for this is not what he meant. He and shaman combine forces, reminding they are the Tenyi Masters - a new Link-3 card. Shoma compares them to the "Gemini Imps", which makes the two cringe why did he compare them to that. Shoma notices that the masters have the combined ATK and Link Rating of the monk and shaman combined.
The monk and shaman confirm that, though the latter belittles the former for being stupid. The monk is a bit astranged, since she meant him and they are supposed to be one card. Regardless, the berserker states Shoma has got all the cards needed to complete his Deck. Shoma is pleased, and places the "Tenyi's Dragon Ogre Deity" in his Deck. Shoma is glad, and when he looks up, he becomes startled to see his sister gazing upon him. His sister is displeased, for she has been watching Shoma build his Deck the entire time and he didn't even notice, for he simply tuned the world out. The next day, Shoma visits the Sattelite Shop, and encounters Strong Jukyu, his friend. While Strong Jukyu is pleased to see him, he advises Shoma to find others to Duel against. Shoma panics, for he does not have any other friends, and becomes mortified when suggested to talk to others.
Strong Jukyu reminds he and Shoma are friends, but states Shoma needs to find more friends to battle against. He thinks Shoma should man up and speak with someone, who is waiting in here to talk to them. Shoma looks ato the battlefield, and sees a lone girl and her White Potan plush. Shoma takes Strong Jukyu's advice and breaths in, then goes to talk to her. Strong Jukyu supports him, but realizes he mentioned a plush. He goes to warn him, but is too late, for Shoma is already speaking with her. The girl looks to Shoma, who extends his hand, asking her for a Duel challenge. A guy behind him comments it is a "moonlight that illuminates the dark light". He mutters that the girl, his sister Tsukiko, is not for the likes of the people of the shop to pass at. The guy, Dark Kuroda, takes Shoma's hand, and challenges him instead.
Tsukiko looks at Dark Kuroda, while Strong Jukyu tries to explain that Shoma is new to this shop, and he didn't know anything about him. Dark Kuroda mutters that Strong Jukyu is with Shoma, and hits his arm. Strong Jukyu tries to explain that Shoma simply challenged Tsukiko to a Duel, to which Dark Kuroda mutters that light and darkness are part of the same coin; to take one away disturbs the balance. Strong Jukyu frowns, and decides not to have anything against this guy speaking nonsense. Strong Jukyu calls Shoma to walk away, who actually wants to battle Dark Kuroda. Strong Jukyu gasps, to which Shoma braces himself for the fight. Strong Jukyu is in shock, and tries to tell Shoma that these siblings are trouble. Shoma simply smiles, so Strong Jukyu has him do his thing, and remarks Shoma to be a greedy Duelist.
Shoma and Dark Kuroda look at each other, while Tsukiko comments that this is a bad idea. At the rooftop, the two sides have their Duel. Dark Kuroda's arm hurts, and declares the time for darkness to trise. Strong Jukyu and Tsukiko wonder what does he mean by that, for it is broad daylight out here. Dark Kuroda Normal Summons "Mathmech Sigma", and lowers its ATK to Special Summon "Mathmech Subtraction". Using these two, Dark Kuroda opens an Overlay Network, to Xyz Summon "Primamathmech Alembertian". Shoma remarks the theme of Dark Kuroda's Deck, who detaches the two Xyz materials to add "Mathmech Addition" to his hand from his Deck. He then tributes the Xyz Monster, Special Summoning "Subtraction" and "Sigma" to his field. Next, Dark Kuroda increases the ATK of "Sigma" to Special Summon "Addition" from his hand.
Shoma observes as Dark Kuroda tunes "Sigma" and "Addition", Synchro Summoning the level 8 "Geomathmech Magma". Strong Jukyu is displeased that Dark Kuroda is taking so much time, to which Dark Kuroda lashes at him. Finally, Dark Kuroda tunes "Subtraction" and "Magma", Synchro Summoning the level 12 "Geomathmech Final Sigma". As his final move, Dark Kuroda equips his boss monster with "Mathmech Billionblade Nayuta". Shoma is amazed by this high-level "Mathmech", and notes as long as it is in the Extra Monster Zone, it is unaffected by Shoma's card effects. He decides to top it with ATK power. Then again, he remarks that the "Nayuta" sword lets "Final Sigma" gain ATK, even when attacked. Shoma takes his turn, and draws "Waterfall of Dragon Souls", and puts it alongside "Tenyi Spirit - Ashuna", two of his "Tenyi Spirit - Mapura", "Jet Synchron" and "Vessel for the Dragon Cycle".
Shoma sees he has to dispose of Dark Kuroda's Equip Spell. He looks at his cards, until Dark Kuroda addresses him. He declares he will destroy his Deck with his right arm for trying to pass at Tsukiko. His arm burns, declaring he won't let Shoma go unpunished for such a revolting thing. As Shoma tries to think what does he mean by that, Strong Jukyu and Tsukiko admit their embarrassment towards Dark Kuroda, who was an elementary school Duel King, but nobody wanted to deal with him because he was always making a big fuss.
Next, he tributes "Primamathmech Alembertian", Special Summoning "Subtraction" and "Sigma" from his GY, then Special Summons "Addion" by its own effect (Level 4/1000/1000) by increasing the ATK of "Sigma" by 1000 ("Mathmech Sigma": 1000 → 2000 ATK). Dark Kuroda tunes the level 4 "Sigma" with level 4 "Addition", Synchro Summoning "Geomathmech Magma" (Level 8/2500/0). Dark Kuroda then tunes the level 4 "Subtraction" with the level 8 "Magma" to Synchro Summon "Geomathmech Final Sigma" (Level 12/3000/0). He then equips the latter with "Mathmech Billionblade Nayuta" Equip Spell Card.
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