In a set of flashbacks, a renowned swordsmith named Masamune is asked to forge a sword of great mystical power. He agrees, but warns that the process will take a year. Masamune’s apprentice Muramasa promises to craft a similarly powerful blade in half the time. The warrior accepts. However, Muramasa’s dark nature becomes bound to his creation, the Sword of Doom, turning the warrior into the Black Samurai. When Masamune completes his own weapon, the Sword of Fate, he entrusts it to the Green Dragon [Brian Cox], and a fierce battle ensues as the Green Dragon fights the Black Samurai, eventually imprisoning him within the Sword of Doom. The Green Dragon hides the weapon and encodes its whereabouts in a riddle etched on the Destiny Scroll, marking the fate of these ancient powers.
In the present day, Tokyo’s historical museum comes alive with tension. Mr. Takagawa [Sab Shimono], the curator, and Kenji [Gedde Watanabe], a diligent janitor, witness the eerie spectacle of the Black Samurai seemingly stepping out of history. The Mystery Inc. gang—Shaggy Rogers [Casey Kasem] and Scooby-Doo [Frank Welker], along with Daphne Blake [Grey DeLisle] and Velma [Mindy Cohn]—arrives at Mirimoto Academy, a prestigious martial arts school near Tokyo, where Daphne has been invited to compete. There they meet Miyumi [Kelly Hu], a close friend of Daphne; Miss Mirimoto [Kelly Hu], the academy’s formidable head; and her bodyguard Sojo [Kevin Michael Richardson], as well as Takagawa’s wary presence. Takagawa warns that the Black Samurai is intent on stealing the Destiny Scroll, a move that could unleash catastrophic plans.
The tournament begins with a twist: Mirimoto reveals she has stolen a copy of the scroll and hands the genuine one to the Mystery Inc. team. As Shaggy Rogers and Scooby-Doo unknowingly solve the riddle, Mirimoto accelerates her scheme to seize the Sword of Doom. The adventures take them to an uncharted island in the Pacific, where they fend off island natives and the Black Samurai’s forces. During the confrontation, the Mystery Inc. gang unmask the Black Samurai as Sojo [Kevin Michael Richardson], exposing the deception at the heart of Mirimoto’s plan. They recover the Sword of Doom and return to Mirimoto, who elaborates on her plan to resurrect the Black Samurai and seize control of the world—using robotic ninjas engineered to mimic the tournament’s fighting styles.
Miyumi betrays the group, siding with Mirimoto, and most of Mystery Inc. and Takagawa are captured. In a desperate turn, Daphne passes the Sword of Doom to Shaggy and Scooby, who escape aboard a jetpack with the aid of their wits and teamwork. The duo are pursued by robots, who manage to seize the weapon, but are rescued by Matsuhiro [Keone Young], a sushi shop owner and seasoned samurai, who offers ancient traditions and disciplined training to the pair. With Matsuhiro’s guidance, the duo journeys to seek aid from the Green Dragon to counter the Black Samurai once more.
Meanwhile, Kenji inadvertently frees Mystery Inc. and Takagawa from captivity, and the two craft an EMP device to disable the robot ninjas. A repentant Miyumi attempts to reclaim the Sword of Doom, but Mirimoto retrieves it to empower Sojo, who frees the Black Samurai. The Black Samurai refuses to serve anyone but himself, defeating Mirimoto and Sojo and subduing the others. Just in time, Shaggy, Scooby, and the Green Dragon arrive to turn the tide, using the Sword of Fate to counter the curse. With Matsuhiro’s training and the Green Dragon’s power, Scooby wields the Sword of Fate to destroy the Sword of Doom, breaking the Black Samurai’s curse and returning him to mortal life. Grateful, the samurai thanks Mystery Inc. before dying, leaving the fate of Mirimoto and Sojo uncertain.
In the aftermath, Mystery Inc. visits the newly renovated museum, where a statue of Scooby stands to honor his role in saving Japan. The team invites Miyumi to join them on their journeys, but she declines, choosing to focus on her own path for the time being.
