Don't ever take what's portrayed in a scifi movie as fact. You can use the fingers on one hand to count the number of scifi movies that go out of their way to be faithful to science. "Mission to Mars was not one of them. (Nor was Total Recall.)
Will you die if you take your helmet off in space? Of course. Your brain needs oxygen, as does the rest of your body. Without oxygen, you will go unconscious rather quickly. Then you'll die shortly later. You won't blow up. You won't instantly freeze. You might however get an ugly postmortem sunburn.
From this "Ask an Astrophysicist" page at NASA, Human Body in a Vacuum
You do not explode and your blood does not boil because of the containing effect of your skin and circulatory system. You do not instantly freeze because, although the space environment is typically very cold, heat does not transfer away from a body quickly. Loss of consciousness occurs only after the body has depleted the supply of oxygen in the blood. If your skin is exposed to direct sunlight without any protection from its intense ultraviolet radiation, you can get a very bad sunburn.