Ratbat and Starscream view the Autobot tapes and before them unfolds the story of an Autobot mission led by Optimus Prime, to Boltax, millions of years ago on Cybertron. The Autobots encounter various traps and obstacles on their way, and unbeknownst to them Megatron is on their trail.
Thunderwing dukes it out with Unicron, but now that the matrix is coated with evil it's power against Unicron is weakened, so Unicron is able to destroy Thunderwing. Blackrock snaps Circuit Breaker back to sanity long enough for her to distract Unicron - this enables Prime to get to the matrix and bring it's inherant goodness back to the surface. He then plunges it into the maw of Unicron, sacrificing himself along with it. Unicron explodes... and the battle is over.
Wheeljack and Prowl find the still alive remains of Prime. He whispers his last words to Prowl, and Prowl ain't happy about whatever it was he said. Elsewhere, HiQ is being chased by a monster. Prowl goes and tells Grimlock that Prime's last words were that the Dinobot leader should take command of the Autobots once more. Grimlock isn't particulary over the moon and goes for a walk with the Dinobots, during which his body freezes up. HiQ runs into them and tries to warn them about his pursuers...
The flashback continues...Optimus Prime meets Boltax and explores the legendary vast Underbase, a source of infinite knowledge and power. Soon Megatron is on the scene, killing Boltax and his disciples and shooting Prime near dead. Prime is left with no option but to unleash the Underbase into space rather than let it fall into Megatron