2001: A Space Odyssey. All-in-all, it is a great classic, and is straightforward. However, as soon as the subtitles announce the segment: "Jupiter and Beyond the Infinite", the last half hour or so of the movie makes zero sense. Of course, everything was explained in the Arthur C. Clarke novel on which the movie is based, but the movie gives no such explanations, and the sequel is really your only source of relief.
Either that or the Final Countdown about the USS Nimitz supercarrier getting sent back from 1980 to December 6, 1941. They spend the whole movie building up to the men realizing they're in the past and can intervene at Pearl harbor, single handedly smashing the Japanese fleet with the most powerful warship of 1980, only to be sent back to 1980 just before the decisive battle begins.