Optimus Prime is the leader of the Autobots, a small band of surviving Cybertronians who fight against the Decepticons who wish to conquer the universe. After losing his homeworld, Cybertron, Optimus has vowed to protect the Earth from any foe.
Optimus came through the space bridge with Bumblebee and landed on Earth. However, the Earth they ended up on was being invaded by Fuse. After searching the globe, they went to Elmore because it wasn't infected yet. With this knowledge, the two Autobots have sworn to keep Fusion activity out of Elmore. They provide missions that allow the player to gain Cybertronian gear as rewards.
Optimus and Bumblebee are on a different Earth from the one they had landed on. While assisting in the war, the two have also been working on building a space bridge of their own to get back to their Earth.