Sam Witwicky, a high school student, buys a second hand car. Only to discover later that it is in fact BumbleBee an Autobot assigned to protect him.

Autobots and Decepticons are two rival clans belonging to the planet Cybertron. The war between them for Allspark, a magical talisman with infinite powers, has destroyed their planet. And the war has now moved to Earth. Autobots are the noble group and Decepticons the evil ones with the desire to use the power of Allspark to rule the universe.

Megatron the leader of the Decepticons came to Earth following AllSpark. But he was frozen into a paralysed state by the cold of the Arctic Ocean. He was discovered by Captain Witwicky and his team (circa 1850) and since then has been kept in a cryogenic state in a top secret facility. The Decepticons have come to the Earth to free Megatron and capture AllSpark, which is also kept in the same facility. Meanwhile the Autobots led by Optimus Prime also reach Earth.

Sam the grandson of Captain Witwicky and his girlfriend Mikaela get involved in this war between the aliens.