Originally posted by: biggestmuff
Read the book. It's perfectly clear what's going on.
Cliffs: Thousands of years ago, an alien race installs a monolith (a large clear one in the book) on earth to spark intelligence in the crude animals living on the human-less planet. The aliens secretly install another monolith on the moon, but this isn't divulged until later in the book and movie. Through generations of evolution, the human race is create. The human race proceeds to develop new technologies, finally inventing space travel and eventually finding the monolith 'alarm system' planted on the moon by the alien race. The humans figure out that the monolith on the moon transmits a signal to Saturn (in the book and Jupiter in the movie). A space mission is created to send a crew to Saturn to investigate. The ships AI freaks out and kills everyone except for one astronaut. This one astronaut, piloting a small vessel, finds where the moon's alarm system signal went...a third monolith floating in space. As he approaches this monolith, a worm hole opens and transports him past alien civilizations to a alien world. However, much like the aliens in Contact, they think the human astronaut would freak out if he saw them and their world, so they create a human 'place' for him to live. The astronaut grows old, dies and then, much like the apes evolved into humans, the astronaut evolves into the next generation of being...pure energy. However, in the movie, this transformation is symbolized by seeing a baby floating in space.
The second and third books are good, but stray from the first one slightly. In the last book we learn that the monolith on the moon was a sort of test for the humans. However, the humans uncovered it too late in their progress of development. The alien race was disappointed by the humans not evolving and developing technology fast enough, so they planned on wiping out the planet. Mankind learns of this, bands together and kills the aliens with a computer virus (I think...it's been a while) with the help from the astronaut/pure energy being from 2001.
I think that's as short and thorough as you can get.