What is going on in transformers 2?

Answer 1

in Transformers 2 Revenge of The Fallen this happens: Megatron was never really in charge of all decepticons, the ORIGINAL Decepticon is known as The Fallen he was one of the original 7 Primes gone bad, and only a Prime can defeat The Fallen. with the original 7 primes, they'd go planet to planet destorying suns, BUT they had one rule: never destroy a sun if it supports life on a planet. but The Fallen tried to defy this rule

the reason they destroy suns is because they supply Energon.

the Primes would use a HUGE machine called the Sun Harvester. the Sun Harvester is turned on with the Matrix of Leadership. 6 of the 7 primes stole the Matrix from The Fallen and hid it in a corner against a mountain in Egypt. and now the only one who knows where it is , is THE FALLEN. now he plans to use it to destroy Earth for its sun.

in the beginning the Autobots are fighting a huge decepticon named Demolishor.

you can hear the whole story on "Transformers Revenge of The Fallen Wikipedia"