Tales of Symphonia/Plot

Plot
Lloyd Irving and his friend, Genis Sage accompany the chosen, Colette Brunel and her guardians, Raine Sage and Kratos Aurion on her journey of world regeneration. The purpose of the journey is to replenish Sylvarant with mana, a necessary energy to their survival. The path of the journey consists of five temples which must be unsealed by Colette. On their journey, they meet Sheena Fujibayashi who comes from Tethe'alla, a world parallel to Sylvarant. She tells them about how the two worlds vie for each other's mana through the journey of regeneration. Hoping to find answers from the angel Remiel, the party meets him at the final seal and are betrayed by Remiel and Kratos; both are revealed to be part of Cruxis, an evil organization led by Mithos Yggdrasill.

The party leaves for Tethe'alla to search for a way to save both worlds and are joined by Zelos Wilder, Presea Combatir, and Regal Bryant. The party learns that awakening the Summon Spirits in both worlds will sever the mana tie between them. Believing that to be the answer, they successfully sever all mana ties between the two worlds. Their actions instead destabilize the Great Seed, the supplier of mana to both worlds, causing it to grow and engulf Sylvarant at an exponential rate. After the party re-stabilizes the seed, they learn from Yuan Ka-Fai about the origins of Sylvarant and Tethe'alla and how they used to be one world but was split by Mithos with the Eternal Sword; the party realizes they can save the world if they wield the Eternal Sword, merge the worlds, and germinate the Great Seed. They decide to confront and defeat Mithos before seeking the Eternal Sword.

Kratos is revealed to have been gathering materials to have Lloyd use the Eternal Sword. He succeeds but Mithos survives his apparent death and possesses a member from the party before fleeing to the comet Derris-Kharlan. Mithos attempts to take the Great Seed with him but is foiled and killed by Lloyd. Using the Eternal Sword, Lloyd merges Sylvarant and Tethe'alla together and germinates the Great Seed into a Giant Kharlan Tree to supply the world with mana.

Plot development
Takumi Miyajima, the game's writer, explained Tales of Symphonia was planned to create a "unique symphonia", dependent on the player's choices and the affection system. Miyajima wrote many scenarios, with the most significant events centered on Zelos Wilder. Originally, Zelos' death was supposed to be canon. His survival would have been dependent on the affection system: He would die early on if he was ranked the lowest in the affection system, would die at the end if he ranked in the middle, and would only survive if he ranked the top. However, the development staff suggested to have Kratos Aurion return to the party and the team discussed how it would work. The change in scenario had Miyajima rework Zelos' death into a non-canon path of the story. While generally a self-contained story, Symphonia apparently takes place within either the same or a similar timeline to the first Tales game, Tales of Phantasia.