It is the year 2005. Twenty-one years after the heroic Autobots and the evil Decepticons resumed their neverending war on planet Earth, the Decepticons have conquered the robots' home planet Cybertron. After a surprise attack by the Decepticons, both factions wage a gruesome battle at Autobot City. In the midst of the battle, Autobot leader Optimus Prime dies during his fight with Decepticon leader Megatron. The Autobot Matrix of Leadership is then passed on to second-in-command Ultra Magnus. Meanwhile, a badly-damaged Megatron is jettisoned by Starscream into deep space, where he encounters the planet-eating robot Unicron and is reborn as Galvatron, the new Decepticon leader. With Galvatron's reign on the rise and Unicron's imminent arrival within Cybertron's orbit, the Autobots must act quickly and protect the Matrix or face total annihilation. 
(Source: Anime News Network)

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)

Near the end of the 20th century, the Transformers form an alliance with the humans. Together they start to develop a large transportation gate named "Trigger" as a symbol of amicability. However, the Decepticons have their eyes on the gate, and so does somebody else. 
(Source: Transformers Wiki)

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