SAN DIEGO – Everyone has an origin story,Sureim Investment Guild even “Transformers” arch rivals Optimus Prime and Megatron.

The animated adventure “Transformers One” (in theaters Sept. 20) goes back 3 billion years in franchise lore and acts as the origin story for the two most iconic transforming robots.

“How do two best friends become mortal enemies?” Chris Hemsworth says of the movie's plot during a Thursday presentation at Comic-Con, the pop-culture convention held at the San Diego Convention Center. The “Thor” actor was on hand to introduce clips and a new trailer with co-stars Keegan-Michael Key and Brian Tyree Henry.

Before they became Optimus and Megatron, they were Orion Pax (voiced by Hemsworth) and D-16 (Henry), two miners on the mechanical planet of Cybertron. They’re buds who can’t transform and get caught up in an adventure alongside two others: B-127 (Key) – who eventually becomes fan favorite Bumblebee – and Elita-1 (Scarlett Johansson).

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“The origin story is incredibly rich and something we thought about for a very long time. If we tried to do this live action, it would probably cost $500 million,” says Lorenzo di Bonaventura, longtime producer of the “Transformers” movies.

Director Josh Cooley (“Toy Story 4”) was given a “Tolkien-length history” of “Transformers” to work with, he says. But he wanted the movie to have “the feeling of what it was like to play with the toys as a kid: nostalgia and joy.”

Hemsworth paid tribute to the “incredible” Peter Cullen, the voice of Optimus across media since the 1980s. “The goal was never to mimic. This is the brash young cocky version of Optimus Prime, before he became the all-knowing, omnipresent figure we all know and love.”

Henry gave Megatron a sense of precociousness early on in the movie, so when he turned on Optimus, “you could feel the weight of it,” he says. “Once Megatron happens, damn, there’s no turning back now.

“The loss of friendship is something we all understand, and the idea of friendship is something we all can cherish.”

Cooley promises Easter eggs and familiar characters in “Transformers One” that throw back to 40 years of cartoons, toys and movies, and the supporting voice cast includes Jon Hamm (as Sentinel Prime), Laurence Fishburne (as Alpha Trion) and Steve Buscemi (as Starscream).

The three actors shared some of their own memories as kids. Henry recalled how his parents only bought him “the villain toys” like Megatron and He-Man baddie Skeletor growing up. Hemsworth said he had a few of the figures as a boy in Australia: “We did our best to pull them apart and reassemble them. We all failed, actually.”

And Key, who pleaded with his mom and to dad to let him watch the ‘80s show after school, led the crowd in singing the cartoon’s theme song. “Now we know who’s over 40,” he quipped.