Transformers 3 takes place three years after the events of the second film. The Autobots, led by Optimus Prime (voiced by Peter Cullen), have been living in peace on Earth alongside humans. However, their tranquility is short-lived as they soon discover that the Decepticons, led by Megatron (voiced by Mark Ryan), are planning to return to Earth. The Autobots soon learn that the Decepticons have discovered a Cybertronian spacecraft hidden on the moon, which contains a powerful energon source that could be used to destroy the planet.