Chris is right, but the reason is subtle, particularly because your description of the story isn't very detailed. We say "the movie tells the story..." when we are talking about one particular plot line of one particular movie. We can also say "the movie tells a story..." when we mean that the movie follows a general kind of storyline that is followed by other movies.
Thus, for example:
"The movie tells the story of a young girl named Dorothy, who travels to the Land of Oz, and meets a scarecrow, a tin man, and a cowardly lion, who help her get the broomstick of the Wicked Witch of the West."
"The movie tells a story of fantasy, adventure, friendship, and self-reliance."