A. Early man is little more than an animal before a mysterious object appears on their planet and signifies the next step forward: The creation of tools, which immediately leads to the creation of weapons, and then we (awesomely) jump-cut to millions of years later. Our first weapon has evolved into our ultimate weapon: A nuclear arsenal poised ominously in outer space.
B. Then we (slowly but very coolly) discover that another mysterious object has been discovered beneath the surface of the moon. When modern man places his hand on the second "monolith," a signal between the moon and Jupiter is opened. Looks like man is officially "ready" for his next step.
C. The most memorable part of the film is the Jupiter journey. That's where two human astronauts are forced to match wits with the ultimate tool: a virtually infallible computer that also happens to have a pretty snotty attitude. What began as a bone has become a super-computer, and it's right about now that mankind has allowed its tools to become just a little too powerful. The computer decides that humans are a variable too volatile too ignore, so in an effort to maintain the Jupiter mission, HAL kills everyone except for one clever astronaut who destroys his ultimate tool just as the journey is ending.
D. Just over Jupiter our one remaining human finds the mysterious object yet again, somehow enters into it, travels through numerous dimensions, and ultimately transforms into the next grade on the evolutionary scale: It's the star-child, baby, and just as the movie ends the being is about to land on Earth.
That's pretty much it, right?