Quick recap
Megatron is a Cybertronian miner when Senator Decimus announces the closure of his mining outpost. At a protest, Megatron kills a guard and escapes with Rumble and Frenzy on a shuttle. In Kaon, the miners fight in illegal gladiator games, armed by Soundwave on behalf of Senator Ratbat. Starscream, Thundercracker and Skywarp join them, and Megatron launches the Decepticons at a rally. Sentinel Prime arrests many followers, but Starscream's testimony was a trap: the Decepticons kill the senators and seize Kaon. In the end, Megatron kills Sentinel Prime in single combat, builds a throne from the remains of his armor and readies a war to rule Cybertron.

