ไคบะกับโมคุบะกลุ้มใจที่เจ้าแห่งเกมส์ทั้ง 3 พ่ายแพ้ให้กับยูกิ ขณะเดียวกันบริษัทไคบะก็กำลังจะต้องแข่งเกมส์กับบริษัท Omega Town เพื่อชิงความลับของเกมส์ที่กำลังจะออกใหม่ พ่อของไคบะคือไคบะ กอนซาบุโร่ก็กำลังจะกลับมาแล้วและกำลังโมโหเซย์โตะมากด้วย ไคบะบอกว่ายังมีไพ่ตายสุดท้ายอยู่นั่นก็คือเจ้าแห่งเกมส์คนที่ 4 ซึ่งเป็นอาจารย์ของไคบะสมัยเด็ก ๆ สอนไคบะเล่นเกมส์ต่าง ๆ มากมาย ในวันที่บริษัทไคบะต้องแข่งกับ Omega Town ไคบะก็ได้ให้อาจารย์ของตนซึ่งเป็นเจ้าแห่งเกมส์คนที่ 4 นี้คือไดมอนลงแข่งเกมส์ Duel Monster ไดมอนก็สามารถเอาชนะได้โดยไม่ยาก ระหว่างทางที่รถของไดมอนกำลังจะกลับยูกิที่กำลังเดิน Shopping กับอันซุอยู่ก็เผลอวิ่งตัดหน้ารถจนรถชน ไดมอนถูกส่งเข้าโรงพยาบาล ยูกิเป็นกังวลมากเพราะคิดว่าเป็นเพราะตน แต่แล้วทุกคนก็ต้องตกใจยิ่งกว่าเก่าเมื่อหมอบอกว่าไดมอนน่าจะเป็นคนตายไปตั้งนานแล้ว อวัยวะทุกอย่างในร่างกายเป็นของเทียมทั้งหมด พอไดมอนรู้สึกตัวก็ชวนยูกิเล่นเกมส์ Duel Monster และเล่าให้ฟังว่าตนเป็นอาจารย์ของไคบะ พอไดมอนเห็นยูกิก็นึกถึงหน้าตาของไคบะสมัยเด็กๆ ที่หัวเราะอย่างสนุกสนานและไร้เดียงสา ไคบะโผล่มาที่โรงพยาบาลและพาตัวไดมอนกลับไป ไคบะ กอนซาบุโร่กลับมาที่ Kaiba Corporation และเรียกหาเซย์โตะทันที ในห้องประชุมไคบะสามารถยึดบริษัทของพ่อบุญธรรมตัวเองได้เป็นผลสำเร็จ ผู้บริหารทุกคนเข้าข้างไคบะทำให้ไคบะ กอนซาบุโร่โกรธจัดแต่ก็ไม่สามารถทำอะไรได้เพราะตนได้พ่ายแพ้แก่เกมส์นี้ของไคบะซะแล้ว ไคบะได้จัดฉากเตรียมให้ไดมอนสู้กับยูกิแล้ว ยูกิกับอันซุกำลังเที่ยวอยู่ที่สวนสนุกและก็เข้าไปดู Monster House กัน อันซุถูกจับเป็นตัวประกัน ยูกิถูกบังคับให้เล่นเกมส์ Duel Monster กับไดมอนด้วยแผนของไคบะ เมื่อยูกิเปลี่ยนร่างเป็นยูกิอีกคนก็สามารถสู้กับไดมอนได้อย่างสูสี ระหว่างแข่งอาการของไดมอนเริ่มแย่ลงไคบะต้องคอยใช้เครื่องมือแพทย์และวิทยาศาสตร์ค้ำจุนอยู่เสมอ รวมถึงช่วยโกงให้ไดมอนเห็นไพ่ยูกิด้วย แต่ไดมอนก็ปฎิเสธพยายามไม่โกงจนวินาทีสุดท้ายไดมอนก็พ่ายให้กับยูกิและขอร้องยูกิให้นำจิตใจที่แท้จริงและรอยยิ้มของไคบะกลับคืนมาที...

Come out! The last and strongest game master

There is an old man sleeping in a capsule and dreamming back to his memory to his past. At Kaiba' mansion, Kaiba was a kid with full of smile. The old man taught Kaiba about playing games. When Kaiba could win a game, he seemed to be very happy. The old man named Dimon much liked pure smile of his young kid.

At Kaiba Corporation President Room, Kaiba and Mokuba are discussing about three game masters defeated by Yugi. Soon , Kaiba Corporation has to play game against Omega Town, rival company. The winner will get the detail of new game available on sale soon. For game profit of Kaiba Corporation, Kaiba can't lose. Mokuba tells his brother that dad is coming back. He seems to get very angry at Kaiba. Kaiba tells Mokuba not to worry. He has already prepared everything. Also, he has the last game master left to play game against Omega Town. At laboratory of Kaiba Corporation, Kaiba releases the last game master named Dimon. Dimon gets up and calls Kaiba as his master. Kaiba assigns a job for him to defeat Omega Town.

In the city, Yugi and Anzu are shopping in a store. Honda tries to guide Miho lovely cloths. Anzu asks Yugi which cloths match her. Yugi says that Anzu looks pretty to every design. On the other side, Kaiba Corporation VS Omega Town is going to start. Mokuba is taking care of Dimon who seems to stay only 3 hours. Both companys have to play Duel Monster card games. The winner will get new game project. Omega Town's player asks if Kaiba Corporation lets him play game against an old man because he won't care what gonna happen with him. Dimon explains that playing game doesn't depend on age but on experience. Then game starts. Dimon can defeat Omega Town very easily. Mokuba sends Dimon to the car and tells the driver to send him back to Kaiba Corporation's laboratory. Back to the city, Yugi and Anzu are walking together. Anzu invites Yugi to Domino fun part. Yugi agrees and points to game store. There is new Duel Monster card game available there. Without caution, Yugi hurries to go across the road. He is almost crashed by a car. Anyway, the car runs out off the road. Though Yugi safe, a person in the car is taken to the hospital. Of course, that person is Dimon who has just finished his game against Omega Town.

At the hospital, Yugi feels responsible for his carelessness that caused the accident. He asks the doctor to save this old guy. The doctor is very surprised because this guy should have died for a long time. Yugi and Anzu are amazed. The doctor says that the guy may survive by some technology. Then Dimon gets up and says not to worry about him. Yugi much apologizes for what he did. Dimon noticed a deck indisde Yugi's pocket. He invites Yugi play Duel Monster game. Yugi wonders if this guy also play Duel Monster card game. Game starts, Dimon summons Sleeping Worm (3/500/1700). Then Yugi summons Dark Mammoth (4/600/300) and then Black Scorpion (4/800/450). Dimon notices Yugi's serious eyes. He summons Gagal (5/1000/1200). Yugi laughes when he knows that he is finally defeated. Dimon suggests that Yugi play without tactic. It is easy to guess. When Dimon sees Yugi laugh, he reminds himself about Kaiba who once ever enjoyed playing game and smiled like Yugi. Then Kaiba and his teams come in to take Dimon back to Kaiba Corporation's laboratory.

At the top of Kaiba Corporation, the president, Kaiba Gonsaburo, has just reached the company by helicoptor. He asks Mokuba where Seto is. Mokuba tells his dad that his brother has been waiting for him at meeting room. All administrations are ready in conference. Gonsaburo draws the topic why Seto hasn't finished game land project that costs three times over budget. Seto replies that in order to build perfect game land, it must take both time and high investment. It is like a game because no one knows what is gonna happen. Gonsaburo is getting more angry. He knows what Seto is gonna do. So, it may be too fast to give Kaiba Corporation to him. Seto says that he can take care of his company. Gonsaburo asks him say again whose company he said. Then Seto sends a signal. All managements there are with Seto. Gonsaburo asks him if he already forgot who brought him up. Seto says that his dad is a loser of this game. Following what he learnt from dad, Seto knows that the loser must be gone. Gonsaburo is shocked and is unconsious. Dimon comes in to greet his boss. He will follow Kaiba's order, no matter what. So, Kaiba assigns him to defeat Mutou Yugi, a young boy whom Dimon met at the hospital. Dimon thinks that he has to duel againt a boy with pure smile. However, for sake of his master, Kaiba, he has to leave his feeling and defeat his boss's rival. Kaiba prepares a special and new wheel-chair for Dimon.

At Domino fun park, Yugi and Anzu are having date. Anzu notices that Yugi look very happy like a kid. She wonders if she would see another smart Yugi again. Yugi invites Anzu to go into Monster House. In fact, he images that Anzu may be so afraid that she hugs him. However Anzu seems to enjoy monsters in Monster House. Yugi wonders what perference of Anzu is. Then he hears Anzu's scream. He first thinks that she may be frightened as he has imagined before. Anyway Yugi is surprised when he sees Anzu being caught by a monster. A block that Yugi is standing on is lifting him up to battle box. Yugi sees Kaiba and the old guy he met at the hospital there. Kaiba tells him if he would like to save Anzu, he has to duel against the 4th game master, Dimon. Teeth of monster will come closer to her. Anzy is much frightened and is unconscious. Yugi changes into Yami Yugi. He tells Dimon that he doesn't hate him but he also has to save his friend. Game between Yugi VS Dimon starts.

First Turn, Dimon summons Skull Bird (3/800/100) with ability to attack by ultra sonic, and faces down a card. Next, Yugi summons King Rex (4/1200/700) and lets it attack. Dimon activates trap card, Golgotha sand, to stick King Rex. Then he lets Skull Bird attack King Rex. With high technology of Kaiba Corporation, this Monster House is improved to simulate vitual images from cards. Dimon tells Yugi about his story with Kaiba. When Kaiba was young, he ever took care of him during his sick. Kaiba gave him an orange with pure smile. He really likes that time. Kaiba tells Yugi to continue the duel. Yugi summons Big Tree (2/400/600) in defense mode and faces down a card. Next Dimon summons Bloody Zombi (3/700). At this time, Yugi activates his trap card, Wonderful Water. The water activates Big Tree to produde seeds. The seeds fall down into water , grow up and destroy Skull Bird. Dimon is getting bad. Also, his health isn't good enough for long duel. Kaiba tries to power up Dimon by special liquid at wheel-chair. The wheel-chair also provides him special telescope to look through Yugi's eyes and see his cards. Dimon refuses this equipment and says that he can win by himself. Game continues. Dimon summons Golden Pegasus and an Angel Alferia Nothing can beat Yugi's monster, Devil Dragon. Yugi wonders why he summonned only low-level monsters. Then Dimon summons his key card, Zombi Master (4/500) that can resurrect monsters from his graveyard. Three monsters from the cemetry are resurrect and combined to attack all monsters of Yugi. Then Yugi activates Regenerate Card. He regenerates an Angel Alferia from Dimon's cemetry. So an Angel Alferia isn't activated by Zombi Master of Dimon anymore because it becomes monster of Yugi. Yugi combines an Angel Alferia with Light Flute and finally defeats Dimon. Anzu is released. Dimon falls down and asks Yugi take real heart of his master, Kaiba, back. Kaiba is moving out from Monster House. A boy runs and accidently bumps him. He drops an orange on the floor. Kaiba stomps on it without feeling. He looks very cool. Next time, it is his turn.