That movie is "Serenity," which is set some time after the last episode of the series. In fact, in the commentary on the new, "Special Edition" DVD release, Joss Wheadon says that the plot line for Serenity was going to come along sometime during the second season of the series. It's a very good movie, and perfectly watchable if you've never seen the series (according to some friends who saw the movie before the series.)
But I can't say enough about the series. I'd guess that since buying the boxed set of DVDs, I've watched each episode at least 8 to 10 times. And they never get old, even though I practically know everything by heart.
Mark