So it seems that Sentinel Prime is a sneaky old robot. Back on Cybertron he organised a deal with Megatron to rendezvous on earth, where together they would bring Cybertron to a new world. Problem is that Megatron mustn't have discussed this with the other Decepticons as they took out the ship, causing it to crash on the moon.
Although Megatron has been secretly hoarding the 'pillar' rods, they still needed to lure Prime into a trap where he frees the Sentinel and revives him, knowing that its only the wise Sentinel who can bring about the space portal.
The Sentinel announces across the world that the Autobots must leave. When they do, they will steal some of earth's resources but then leave in peace. The Government agrees.
Patrick Dempsey kidnaps Sam's girl, and locks a robot watch onto Sam's arm to find out the autobot plan of attack. Prime accepts his mistake in trusting the wrong robot, and tells Sam they will leave without a fight. They get into the spaceship and blast off -- but Starscream has been waiting and launches missiles at the craft so it explodes in a ball of flame.
The Decepticons turn nasty. A whole load of them are brought in from an unknown place (it's not clear where or really how) and they cause havoc on earth vaporising humans with 'Mars Attacks' style death rays.
Sam decides to free his girl, and enlists the help of the dude from 2 Fast 2 Furious (Epps). They march into Chicago, but are scared off by a few Terminator Salvation style robot-helicopter droids. Suddenly the Autobots arrive on the scene and take out the droids. They were hiding in one of the fuel tanks and crash landed in the ocean.
Sam is given a lift to Dempsey's apartment on Trump Tower. He rescues Carly, but is attacked by Soundwave's bird (who is finally defeated when his head is blown off) and they crash onto the street.
In one of the downed unmanned droids (from the good guys) they find a camera to relay intel back to the base about what is going on - and Carly tells Lennox and the others where to find the main red portal pillar. Tomahawks are sent in, and men are recruited to parachute into the drop zone.
Prime finally makes an appearance, but loses his jet pack carriage and needs to go and look for it.
The Seals are dropped in, led by Lennox, fly around in those Batman jump suits, and land on the ground next to Sam. They coordinate an attack, but it doesn't seem the greatest plan in the world and they just end up hiding.
With one missile, Sam's guys get to a glass building to take out the main red pillar rod. Shockwave finds them, and with his centipede tail chews the building in half, and then goes about destroying it while Sam and his boys try to survive.
Finally Prime shows up with his jetpack and sword and chops the centipede in half. Sam's team have forgotten all about missile they had by this time.
Sam is found by Starscream, but single-handedly defeats him using the professor's gadgets, taking out his eyes. He stabs him with a bomb, and frees himself in time before it explodes and this bad guy is finally defeated.
The German tech man (Steve the Pirate) hacks into the mainframe and lowers the bridge. It seems the parachutists landed on the wrong side of the water. Now everyone can get a little closer to the action.
The autobots haven't been seen for ages. It turns out that they've all been captured. The pillars are activated and Cybertron starts to appear above earth. The two tiny tobots hitch a ride in the drone shuttle and start to cause mayhem. The Decepticons start to kill hostages - starting with the professor. Bumblebee is next, and resigned to defeat. Suddenly vehicles drop down from the shuttle above. Bumblebee can now suddenly defeat the kidnapper and defeat all the others guarding them even though the situation hasn't really changed. The shuttle goes down, in some water, but the little robots know they won't make it.
Prime shows up and takes out a pillar. The portal is stopped. Prime battles with Sentinel again, and is losing - again. Sentinel calls on the others to attack Prime. Finally the good guys have a plan ready - for snipers to take out the Decepticon's eyes. The Tomahawks finally show up too - taking out the enemy drone shuttles.
Sam confronts Patrick Dempsey near the main pillar. They fight. Finally Patrick is thrown against the pillar and is electrocuted. Bumblebee then destroys it.
Cybertron's portal is halted again - and this time for some reason it gets messy and implodes in on itself.
Megatron is taking a breather when Carly finds him and appeals to his evil side. She lets him know who really is in charge. Megatron lays into Sentinel and offers a truce to Prime. Instead Prime tears out Megatron's spine (!) and then, with Sentinel on the ground pleading for mercy, points a giant robot shotgun to his old leader and takes him out.
Everyone is happy. Except the odd million people who were vaporized. Or had apartments in Trump Tower.
In the credits, Simmons grabs the Secret Service lady for a kiss. He is promptly sent to prison, but tells them that it was worth it!!