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)

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)

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)

An evil race of transforming robots known as the Destronger make their appearance known by invading a world exposition in New York and abducting Professor Onishi, a renowned scientist. The professor's son Yuuki befriends Fire Convoy and the heroic Cybertron forces, who will stop at nothing to rescue the professor and protect Earth from the wrath of Gigatron and his Destron henchmen.
 
(Source: Anime News Network)

As 2006 settled in, Earth had been rendered completely devoid of Transformers, and the rebuilding of Tokyo was underway. However, Galvatron's impact was revealed to have had a much greater effect than anyone could have predicted&mdash;the blast caused by his impact had scattered his Unicron-empowered cells throughout the planet's atmosphere. When they came into contact with other animals and objects, from marine life to cars, they fused with them, becoming one of many monstrous creatures referred to as Legion. This catastrophe also created the means to defend the world against the Legion, however&mdash;when Galvatron's cells came into contact with human beings, they also become able to fuse with other entities containing his cells by kissing them. With their Autroopers being the perfect candidates for this "ParasiTech" fusion process, the EDC began recruiting and training these "Kiss Players" as combat squads to battle the Legion. 
(Source: Transformers wiki)

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