Transformers: The Beast Wars (note the "the" in the title - it can make all the difference) is a fanfiction to be written by Darthrai and others, serving as a fan-created sequel to Transformers: Revenge of the Fallen and a hypothetical version of the upcoming third installment in the live-action film series. It draws heavily from the Beast Wars TV series, but still incorporates most of the elements from the previous movies in order to feel "familiar".
In the story, Megatron travels back in time to prehistoric Earth in order to prevent the Autobots from existing in the first place, and uses both Decepticons from that time period and from the modern day to form the Predacons. Optimus Prime must travel back in time as well to stop him, and forms the Maximals. "Meanwhile", in the present day, Starscream takes advantage of Megatron's absence and, against his master's orders, kidnaps Sam Witwicky and gathers up those Decepticons that never really liked their old commander and plots to rise against the Predacons in order to take command of both armies himself.