Convinced that the danger had passed, the Autobots nevertheless leave behind on Earth a small group of defenders led by Optimus Prime. The precaution proves not to be in vain — three years later Megatron returns and brings with him Dark Energon, with which he plans to exact revenge for the past defeat. The Autobots are once again forced to take up arms. They have befriended a few Earth teenagers, and now a team of robots and humans fights to protect the Earth from the Decepticons.