The Constructicons steal numerous energy disks from a construction site to make a device that will allow their creator, Megatron, to defeat Optimus Prime by using his only weak point: his honor.
Megatron plans to infuse himself by using the Constucticon's device and the Decepticon's power chip rectifiers to challenge Optimus Prime in a battle of honor with a Cybertron law: the winner can remain where ever he wants, the loser must leave the planet in exile. According the law, they are supposed to enter the battle without aid, but Megatron breaks the rules when he infuses himself with all the Decepticons power. The Constructicons dig a tunnel beneath the Ark, intending to destroy Teletran-1 before it can reveal Megatron's secret.
With all the Autobots and Decepticons as the audience, Optimus Prime and Megatron starts their fight. Due to Megatron's incresed powers, he easily defeats Optimus Prime who inturn orders the other Autobots to return to the base to prepare for their exile.
Back at the Ark, the Constructicons engage the Dinobots in battle but lose. To even the odds, the Constructicons merge into their combined form, Devastator. The Autobots return to base but can't help the Dinobots due to the Cybertronian laws. Inside, Teletran-1 inspects Optimus´s wounds and informs him of Megatron's trickery. With the deal now null and void, the Autobots rush to the Dinobots aid. Hound projects a hologram of a massive Autobot, confusing Devastator long enough for Optimus to fire a shot that breaks him into the individual Constructicons.
The Autobots force the Decepticons back and send them tumbling into a crevice filled with hot lava. Optimus Prime reserves Megatron for himself. Without the powers of his Decepticons, Megatron can do nothing against Optimus Prime who in turn defeats him, sending him to joing the other Decepticons in the lava.