Yuma Tsukumo is an ambitious student who is underachieving in both school and in duels, but can never turn down a challenge. One day, he and his friend Tori Meadows spot their other friend, Bronk Stone, get beaten in a duel against Ryoga "Shark" Kamishiro who takes his deck as a result. When Yuma stands up to him, Shark breaks the key he got from his parents and kicks half of it away, though he gives him a chance to put his own deck on the line in a duel to win back Tetsuo's deck. Noticing Yuma looking less ambitious but still willing to fight against Shark, Bronk manages to return the second half of his key to him. The day of the duel between Yuma and Shark comes, and Yuma's amateur duel skills put him at a disadvantage against Shark's deck. As Yuma defends himself against Shark's assault, his key suddenly repairs itself and he finds himself in front of a strange door, which he opens with his key. Upon returning to the real world, Shark is possessed by a weird force and summons the Xyx monster Number 17: Leviathan Dragon, while Yuma is greeted by a mysterious spirit. [+]more .