Several years ago, high school students Sam Witwicky (SHIA LaBEOUF) and Mikaela Banes (MEGAN FOX) found themselves in an unexpected battle between shape-shifting, extraterrestrial robots. The good Autobots, led by Optimus Prime (voice of PETER CULLEN), were trying to find a powerful cube known as the Allspark, and hoped to do so before their enemies, the Decepticons, led by Megatron (voice of HUGO WEAVING), used the same to turn ordinary Earth-based electronics into robots that would rid the planet of Earthlings.
With the help of military figures such as Captain Lennox (JOSH DUHAMEL) and Sgt. Epps (TYRESE GIBSON), Agent Simmons (JOHN TURTURRO) of the U.S. Government's top secret Sector Seven, and various Autobots -- such as Bumblebee (voice of MARK RYAN) who has the ability to near instantaneously convert into Sam's sports car, like the other Transformers with similar powers -- the good guys prevailed and Megatron was sunk deep into the ocean.
Now, as the humans and Autobots have formed an alliance to keep Earth safe from the Decepticons, Sam's saying goodbye to both his parents, Ron (KEVIN DUNN) and Judy (JULIE WHITE), as well as Mikaela as he heads off to begin his freshmen year in college. There, he meets his high tech roommate, Leo (RAMON RODRIGUEZ), but before the day is done, Bumblebee and Optimus Prime have arrived, with the latter asking for Sam's help.
It seems that one of the original Prime Transformers, The Fallen (voice of TONY TODD), has arrived and wants vital information that now resides inside Sam's head from his brief contact with a leftover sliver of the Allspark. With Mikaela joining him, Sam goes on the run with the aide of Bumblebee and jive-talking Autobots Mudflap (voice of TOM KENNY) and Skids (voice of TOM KENNY), and some unexpected help from ancient Transformer Jetfire (voice of JOHN TURTURRO).
From that point on, another massive battle looms on the horizon, with the U.S. military -- despite being hampered by the arrival of presidential liaison Galloway (JOHN BENJAMIN HICKEY) -- battling alongside the Autobots in hopes of defeating The Fallen and the recently resurrected Megatron.