The Story 

(with as few spoilers as possible;  a longer, spoiler-filled version can be found here.)

    Final Fantasy X follows the story of Tidus and Yuna as they lead a journey to defeat the whale-like monster, Sin that has plagued Spira for over 1,000 years.  Tidus is a star blitzball player from the sleepless city of Zanarkand.  He is transported by Sin to Besaid, Spira, seemingly 1,000 years in the future.  There he meets Yuna, a summoner beginning her pilgrimage, and her guardians (bodyguards), Wakka, Lulu, and Kimahri.  As Tidus' hopes of going home lies along Yuna's route, Tidus joins the travel party and later becomes another of Yuna's guardians. 

     Along the way, the travelers meet Auron and Rikku who also become Yuna's guardians just before Seymour, one of the political, religious leaders, asks Yuna to marry him.  This proposal leads to a series of events that cause Yuna to be put on trial for treason where she learns from Grand Maester Mika that the summoner's pilgrimage is nothing more than a band-aid against the gaping wound of Sin, and the hope that the pilgrimage will one day vanquish Sin is a lie.  

     Once Yuna and her guardians escape their death sentence, they continue their travels along the summoner's pilgrimage as they search for the true way to defeat Sin.  Their journey causes them to commit further treason in the government's eyes, and they are forced to face Seymour in battle many times.  Finally, Tidus' role in Sin's ultimate defeat becomes much clearer, and the characters are then able to vanquish Sin.