Transformers One is an animated prequel that explores the origins of the Transformers, an alien race of metallic humanoids that can transform into vehicles. It delves into the roots of the conflict between the franchise's rival groups: the Autobots and the Decepticons.

Chris Hemsworth may not have been every Transformers fan's first choice for Optimus Prime, but there was one important reason Peter Cullen wasn't right for the job. Spoilers ahead.