In "Revenge of the Fallen", Sam Witwicky has visions of strange symbols and is haunted by Decepticons. He and his friends are kidnapped by Megatron, who is convinced that Sam's visions will lead them to a new "energon" source. Decepticons seek to free the Fallen and destroy Sam while the Autobots and their allies try to save the world.
The Primes originally created Sun Harvesters to produce "energon." Since these machines destroyed stars, the robots agreed to avoid suns with inhabited planets. One Prime, referred to as The Fallen, ignored this directive. Deceiving his fellows, he raised an army, attacked the ancient humans of Earth and built a Star Harvester on the planet. The other Primes defeated and imprisoned the Fallen, who has vowed to destroy the Autobots and Earth.
Sam's visions of Cybertronian symbols convince him of an impending war between Decepticons and Autobots, making it difficult for him to cope with college. He reaches out to Mikaela and when she comes to visit, they are both captured by Decepticons.
The Decpticons are convinced that Sam's visions are the key to finding a new energon source. They revive Megatron, who locates the Fallen. Optimus Prime, Sam and their allies stop the Fallen before he activates the Sun Harvester and destroys the Earth.