I think i just found out the story.
People go to a planet where they meet one of the engineers, only he stayed there waiting for the time to come when humans can reach there...here comes some booring explanation about our history and about the technology and about the knolewdge what he shares with us and so on...then the engineer wants to leave with all his preacious stuffs, but the military part of the crew is ordered to get everything from him and takes over the control of prometheus and shot the ship down, it falls back to the planet then hell breaks loose because the engineer unleashes something in his last moments to kill all the humans and sends a message to the others to bring a ship with full of alien eggs to earth...thats it...
By the way, do you know where can i read that leaked script about the movie? Is it any good?