Sam Witwicky and his new girlfriend, Carly, join the fray when the evil Decepticons renew their longstanding war against the Autobots. Optimus Prime believes that resurrecting ancient Transformer Sentinel Prime, once the leader of the Autobots, may lead to victory. That decision, however, has devastating consequences. The war appears to tip in favor of the Decepticons, leading to a climactic battle in Chicago.