With the Inter-high over and as the end of summer approaches, Sakamichi trains intensely with Imaizumi Shunsuke and Naruko Shoukichi so he can carry on Team Sohoku’s spirit of supporting each other entrusted to him by Kinjou Shingo, Makishima Yusuke, and Tadokoro Jin, the third years who will be graduating before long. One day, Team Sohoku receives an invitation to the Kumamoto Fire Country Mountain Race, an invitational race for teams that performed well at the Inter-high, and their captain Kinjou decides their core Inter-high team will attend.

(Source: Crunchyroll)

Obsessed with revenge against the King of Games, Seto Kaiba commences an excavation in Egypt to find the fragments of the Millennium Puzzle, which once retained the spirit of the "counterpart Yuugi." However, a cloaked man steals two pieces of the Puzzle and disappears before it could be reconstructed. Meanwhile, now in their final year of high school, Yuugi Mutou and his friends plan for their futures, unaware of the danger lurking close by. 

Yu☆Gi☆Oh! The Dark Side of Dimensions tells the tale of vindictive forces distorting reality in whatever way they please. Will Seto and Yuugi move past their differences and save the world, or will Seto's own greed get the better of him?

[Written by MAL Rewrite]

"Blue Eyes White Dragon will bring victory" while "Red Eyes Black Dragon will bring the potential of victory". A shy duelist stumbles upon the legendary Red Eyes Black Dragon, with that card in his possession, he must learn to bring out his confidence to duel. (Source: ANN)

While riding with Jack Atlas and Crow Hogan, Yuusei Fudou's Stardust Dragon is captured by Paradox, a mysterious Turbo Duelist from the future, during a Turbo Duel and turned into a Sin Monster. With the help of the Crimson Dragon, Yuusei chases after Paradox as he enters a time slip, ending up in the past. During this time, Paradox duels against Jaden Yuki, who is still able to use the powers of Yubel and The Supreme King. However, by this time Paradox had also captured Cyber End Dragon and Rainbow Dragon and overwhelms Jaden. He is saved thanks to Yuusei and the Crimson Dragon. Jaden informs Yuusei of Paradox's true intentions. By stealing various monsters from across time and turning them dark, he plans to kill Maximillion Pegasus, the creator of Duel Monsters, preventing the game from being created and causing the events of all three series to never happen. Yuusei and Jaden agree to pursue Paradox, which leads them to the past and causes a meeting with the King of Games, Yuugi Mutou. However, by the time Yuusei and Jaden arrive, Paradox had already attacked his time, supposedly killing both Pegasus and Yuugi's grandpa, and had also managed to steal Blue-Eyes White Dragon and Red-Eyes Black Dragon. After explaining everything to Yuugi, he agrees to fight with Yuusei and Jaden against Paradox in the ultimate three-on-one duel to free the trapped monsters and save both the world and time itself before it's too late. (Source: Wikipedia)