Quick recap
In 2005, the Decepticons rule Cybertron. A raid on Autobot City on Earth ends in a duel between Optimus Prime and Megatron: both are mortally wounded, and Optimus passes the Matrix of Leadership to Ultra Magnus before he dies. Starscream dumps Megatron into space, where he's found by Unicron, a planet-sized robot that eats worlds. In exchange for a new body, Megatron becomes Galvatron and promises to destroy the Matrix, the one thing that can stop Unicron. Galvatron kills Starscream, retakes command, kills Ultra Magnus and seizes the Matrix. At the end, Unicron swallows Galvatron and the Matrix. Hot Rod flies inside the giant, opens the Matrix, becomes Rodimus Prime and destroys Unicron from within. Its severed head is left orbiting Cybertron, back in Autobot hands.

