SPOILER
Megatron is revived and under the instructions of the Fallen. The only remaining original Transformer goes looking for the key of leader ship which will activate a machine which will destroy earth's sun and turn it into energon.
Sam knows stuff about it because of his exposer to the Cube from the last movie and he has to find it while evading the decepticons and athuorities when Megatron outs the Transformers and demand that Sam be turned over to him.