"Tales of Symphonia the Animation: Tethe'alla Episode" is a direct continuation of the "Tethe'alla Chapter" and adapts the second half of the "Tales of Symphonia" video game's storyline. The narrative centers on Lloyd Irving and his companions as they journey through the world of Tethe'alla, a parallel realm to their own, Sylvarant. The group seeks to regenerate the world by uniting the two lands, which are locked in a cycle of decline and prosperity due to the actions of the ancient Kharlan Tree.

The story begins with the party arriving in Tethe'alla, where they encounter new allies and adversaries. Their primary objective is to awaken the remaining Summon Spirits and complete the journey of regeneration. Along the way, they uncover deeper truths about the world's history, the role of the Chosen, and the machinations of the enigmatic organization, Desians, who oppress half-elves and manipulate events for their own gain. Key characters include Lloyd, a determined swordsman; Colette Brunel, the Chosen of Sylvarant; Genis Sage, a young mage; Raine Sage, a scholar and Genis's sister; and Kratos Aurion, a mysterious mercenary with ties to the group's past.

As the group progresses, they confront moral dilemmas and personal struggles, particularly regarding the cost of their mission. Colette's transformation into an angelic being and the sacrifices required to restore the world weigh heavily on the party. The narrative also delves into the backstories of characters like Sheena Fujibayashi, a summoner from Mizuho, and Zelos Wilder, the Chosen of Tethe'alla, whose loyalties and motivations are tested. The climax involves a confrontation with Mithos Yggdrasill, the leader of Cruxis and the true orchestrator of the world's suffering, revealing the tragic origins of his actions and the cyclical nature of the conflict.

The series maintains a focus on themes of coexistence, sacrifice, and the consequences of idealism. It balances action, emotional depth, and world-building, staying faithful to the source material while condensing its expansive narrative into a cohesive anime adaptation. The Tethe'alla Episode serves as a conclusion to the overarching story, resolving the fates of the characters and the worlds they strive to save.