Sentinel actually betrayed the Autobots and made a deal with the leader of the Decepticons to join forces. They arranged to meet in a cafe, but neither of them could fit through the door. The manager kicked them out and Megatron crashed into the polar ice caps of Earth, and Sentinel crashed on the moon.
A few years after Megatron awoke and was freed by his Decepticons, Sentinel awoke too. He fled the moon and met up with Optimus, who still beleived Sentinel was a goodie. Optimus tried to convince Sentinel to help him win the war, but Sentinel was paranoid that Optimus would betray him, even though he had already betrayed Optimus. He fled, and Optimus soon found out about his betrayal.
Sentinel met up with Megatron and conquered the American city of Chicago. Optimus and his Autobots arrived to stop them, and the three leaders engaged each other in battle. At first, Megatron and Sentinel were buddies, but Sentinel soon realised he didn't really need old Megsy any more, so he took over the Decepticons. Enraged, Megatron decided to attack Sentinel.
In the meantime, Optimus and Sentinel had begun their fight. Optimus couldn't break through Sentinel's awesome sword 'n' shield of Spockness and soon lost an arm. However, as Sentinel was about to kill Optimus, Megatron shot him in the back, ripped out three of his ribs and knocked his little hat off. Despite offering peace, Megatron was then killed by Optimus. Finally seeing the error of his ways, Sentinel begged Optimus for forgiveness. To no avail. He was two-shotted by Optimus using Megatron's Shotgun, which was then thrown to the ground in disgust. The 'Golden Age' had ended.