Following the devastating events of Chicago in Dark of the Moon (film), humanity no longer trusts the Autobots and forces them underground in an attempt to wipe them out entirely. Government officials led by an arrogant engineer named Joshua have created their own robots and spaceships similar to Transformers in because of the discovery of a metal which they carbonize and galvanize in an attempt to replace them. Some of these transformers are Stinger and Galvatron.
Four years later, a single father and mechanic named Cade Yeager purchases a rusty old truck so that he may scrap it and sell its parts to obtain enough money to send his daughter Tessa to college. When the truck seems unable to be scrapped, Cade opens the front door and discovers gigantic bullet casings in the front seat. After hearing strange sounds after hooking it up to an electric circuit, he soon realizes that he has stumbled upon a lobotomized yet alive Autobot - Optimus Prime.
Not long after, a paranoid government official who leads the charge against the Autobots sends several of his men and Galvatron to kill Cade, Tessa, and Tessa's boyfriend Shane so they may recover Optimus. Just then, Optimus springs to life, defeats the humans, and takes the three to safety. But soon a robot named Lockdown captures Tessa and Optimus Prime who was fighting Galvatron.
The man-made transformers become sentient. Now it's up to Cade, Tessa, and Shane to ally themselves with the Autobots and defeat humanity's greatest failure.