The Autobots are now personas non grata, hunted down by the CIA, backed by a decision from the upper echelons of power that humans do not need aliens for protection. When a broke inventor in the American countryside runs into the "hibernating" Optimus Prime, the CIA and the bounty hunter Decepticons gun for them. The clash between the machines and the humans, with an underlying sinister plot to "mine" the autobots for their valuable metals, finally reaches its explosive climax in the gorgeous canyons and dingy apartments of Hong Kong.
On the positive side, the 3D is pretty good, and some of the locations and the photography is beautiful. On the flip side, the movie stretches far too long, the obsession over the transforming Autobots and Decepticons gets far too indulging, and you desperately hope for a quick end!
My recommendation - watch it on BluRay, and make liberal use of the FF button!