This game is a parallel story that happens alongside the main plot of Final Fantasy X. The world is called Spira, and Sin is destroying settlements. The world is looking for a summoner to bring another Calm to the world. Most machina are banned under the Yevon religion, but some simple machina can be used. Not everyone follows the Yevon religion, and some choose to use machina for destructive purposes. The world is inhabited by monsters that have formed from dead spirits left unsent.
This is the untold story of a lesser known set of heroes.