Beginning with a recap of the coming of the Transformers to Earth and the story of Devastator, the OVA then gets its original story underway, as the Autobots are shown to be in the midst of constructing the powerful "Scramble City," overseen by their newest arrival, Ultra Magnus. 
When the Decepticons learn of this, their combiner robots are deployed to attack, and a battle between them and their Autobot counterparts ensues. 
(Source: Wikipedia)

Fleeing from the war that has raged on the planet Cybertron for millions of years, the heroic Autobots led by Optimus Prime encounter a surprise attack by the evil Decepticons led by Megatron. The ensuing melee aboard the Autobot ship ends when the ship crashes on a volcano. Four million years later, an earthquake reactivates the ship`s computer and resurrects both the Autobots and Decepticons, resuming the great war of the Transformers. 
(Source: AniDB)

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