It's nice when a movie reviewer can get all the room she wants to write a review. However, most websites and print media have space limitations. Some want film reviews as short as 300 words. Reviewers working under those restraints have to be good self-editors who get the basics of the film and their opinion in while still telling a "story."
Start with a quick summary of the film and your opinion. This should take up the first paragraph and should not be more than three sentences. Identify the name of the film in this paragraph and any important association the audience may have. For example, a woman is far more likely to read a review of the baseball film "Money Ball" if it's introduced as Brad Pitt's new film "Money Ball."
Use the next paragraph to summarize the plot or expand on your opinion. This graph needs to be a few sentences long, but edit out anything inessential. Do not explain more of the plot than is necessary for the reader to understand that review. You do not need to lay out the full details of the plot in a short review.
Use the next paragraph to give credit to something good in an otherwise poor movie, or to say something that a good movie could have done better. This graph is optional, but it does suggest to the reader that the writer approached the film with a critical mindset. The most useless reviews, short or otherwise, are those written by a fan (or anti-fan) who only insults or praises the film and does not explain "why" he felt that way.
Close the review with an anecdote or wrap-up that also summarizes why you either liked or disliked the movie. A closing graph may also point out exceptional performances by the actors, a director, or a cinematographer.
Never give away the ending of a film in any review. Nothing angers film fans (or editors) more.