The saga of ''a nobody crew aboard a crappy little spaceship'' is how Buffy the Vampire Slayer creator Joss Whedon summarizes his feature-film directing debut though the full story behind Serenity is about a thousand words more complicated. The $45 million sci-fi Western, a revival of Whedon's '02 TV series Firefly starring the show's original cast, focuses on that crappy ship's most troubled nobody, River (Summer Glau), a damaged teen prone to psychic flashes and flurries of violence thanks to mysterious government experimentation. The relatively unknown Glau who underwent three months of martial-arts training to get ready for the role wept when Whedon told her River would live again...then had a scary thought. ''I said, 'If you have to recast me with Winona Ryder, or some kind of ''star,'' tell me. I'm ready for it,''' says Glau. ''But no, he wanted me, thank God.''