"Bakugan Battle Brawlers: Gundalian Invaders" is the third season of the "Bakugan Battle Brawlers" anime series, continuing the adventures of Dan Kuso and his friends as they face new challenges in the Bakugan universe. The story shifts focus to the conflict between the Gundalian and Neathian worlds, two warring factions seeking to control the powerful energy source known as the Sacred Orb. The Gundalian Empire, led by Emperor Barodius, aims to conquer Neathia and harness the Sacred Orb's power to dominate the universe. Neathia, a peaceful planet, relies on its guardian Bakugan and the Battle Brawlers to defend against the Gundalian threat.

Dan and his team, including Shun, Marucho, Julie, and newcomer Ren Krawler, are drawn into the conflict when they encounter Fabia Sheen, a Neathian princess seeking their help. The group travels to Neathia, where they learn about the ongoing war and the importance of the Sacred Orb. As the Battle Brawlers align with Neathia, they face off against the Gundalian forces, including Barodius' elite warriors: Sid Arcale, Lena Isis, and Zenet Surrow. Each Gundalian warrior commands powerful Bakugan, such as Linehalt, Aranaut, and Krakix, making them formidable opponents.

Throughout the series, the Battle Brawlers uncover deeper layers of the conflict, including the Gundalian Empire's internal struggles and the tragic backstory of Ren, who is revealed to be a Gundalian spy sent to infiltrate the team. Ren's loyalty is tested as he grapples with his identity and the bonds he forms with the Battle Brawlers. The narrative explores themes of trust, redemption, and the consequences of war, as characters on both sides confront their motivations and allegiances.

The season culminates in a climactic battle for the Sacred Orb, with Dan and his team facing Barodius and his ultimate Bakugan, Dharak. The resolution of the conflict hinges on the Battle Brawlers' ability to unite their strengths and overcome the Gundalian Empire's overwhelming power. The story concludes with the restoration of peace between Gundalia and Neathia, though the characters are left to reflect on the sacrifices made and the lessons learned during their journey.