When Sam (Shia LaBeouf) finds the Matrix of Leadership, at first it turns to dust in his hands.
After being critically injured, Sam sees the Primes of the past, who tell him that the Matrix of Leadership can never be found, it must be earned. The Matrix reforms itself and Sam uses it to revive Optimus Prime.
As soon as Optimus is back on his feet, The Fallen (voice Tony Todd) takes the Matrix to activate the Sun destroyer in the pyramid. Optimus realizes he cannot take on Megatron and The Fallen alone, so Jetfire offers his assistance, and the two merge.
Jet-Optimus destroys the device and The Fallen. Starscream encourages Megatron to retreat, and Megatron does, with the promise, "This isn't over."