Long ago, on the distant planet of Cybertron, OPTIMUS PRIME’s mentor, SENTINEL PRIME, sought to stop the war between the AUTOBOTS and DECEPTICONS—but his ship was shot down before his mysterious cargo could bring peace… or so the AUTOBOTS thought. When a component of SENTINEL's ship mysteriously shows up on Earth, OPTIMUS PRIME discovers that his old mentor may be far from gone. But MEGATRON and his DECEPTICON forces may have their own plans for SENTINEL PRIME.