Basically, Beast Wars starts off 300 years after G1 ends. The Maximals (descendants of the Autobots) and Predacons (descendants of the Decepticons) have had an uneasy peace for the past few generations, during which, they've had to reduce the sizes of their bodies to be more Energon-efficient. This all changes when the new Megatron finds a secret message encoded on a relic - the Golden Record, carried by one of the Voyager probes. This message was put there by G1 Megatron as part of a contingency plan, should the Decepticons lose the war. The message that G1 Megatron left was one where he revealed to any Decepticon descendants who would find it that he knew that the experimental transwarp drives being developed on Cybertron could be used as time machines under the right conditions. And the current Megatron sets out to create those conditions as part of a plan to alter history so that the Decepticons win the Great War.
Unfortunately for him.... his science officer and literal spawn of Unicron, Tarantulas is using him for his own agenda...