Answer:
It's just the sequel to final fantasy XI from tidus being kiddnapped and its just yuna , riiku, and pain ( the new girl) they're disgused as sphere hunts trying to find tidus and spheres because she has given up her master summoning so she won't be hunt and for tidus. sorry to spoil it
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There is no Final Fantasy XI 2. XI is the online game and it has a few expansions, just like any MMO. If you mean FF X-2 (which is what I and the poster above me think you mean), then it is the direct sequel to Final Fantasy X. It takes place two years after Tidus's death (I don't know where that guy got that he was kidnapped...) and follows Yuna after she joins a sphere hunting group called the Gullwings. Sphere hunters search for spheres that may detail some of Spira's history that was wiped clean by Yevon in an attempt to help rebuild the failing government. Though most of them are just in it for the fun. The plot mostly follows the political upheaval in Spira after Yevon falls in the original game, but a person who looks quite a bit like Tidus comes into play as the story progresses.