The main story is told over 10 chapters in Campaign Mode. The first 5 chapters you play as the Decepticons, with the next 5 continuing the story as the Autobots. The Campaign mode can also be played in a Online Co Op mode which I highly recommend. A good friend of mine played the Co Op mode with me on the hardest difficulty and we had a blast playing through the game together.
You are allowed to play either Campaign first, however I suggest playing the Decepticon one first as you will be confused by the story if you don’t. Each of the 10 missions gives you the option to play as 3 different Transformers, the AI will control which ever one’s you don’t choose and assist you in the mission.
The Decepticons story revolves around Megatron’s ambition to control Dark Energon to over take the Autobots and bring Cybertron back to the Golden Age. In the first chapter you use Megatron, Brawl and Barricade and attack Starscream’s research station. Starscream is so impressed by Megatron’s power he asks to join him and learn the power of Dark Energon.
Other Decepticon missions have you playing as Starscream, Thundercracker and Skywarp to reactivate a Dark Energon bridge. You also attack the Autobot Capital City, battle Zeta Prime and have a nasty epic Boss Battle with Omega Supreme. I enjoyed the Decepticon Campaign the most.