V, the only character with sufficient magnetism to hold the narrative together, drops out for an extended period while Evey endures a hellish imprisonment that's contrived in more ways than one, and in the end awfully silly.
Natalie Portman, as skillful as she is attractive, does have her moments -- it's affecting to see her hair being shaved, like Joan of Arc -- but wide-eyed Evey whimpers endlessly, and tediously, on her way to becoming a fearless woman who's able to love.