Headmasters starts with Galvatron leading a new batch of Decepticons to wage war on planet Cybertron with the Decepticon Headmasters. The Cybertronians are rescued by the arrival of the Autobot Headmasters, led by Fortress (Cerebros, the head of Fortress Maximus). It is revealed that Fortress left planet Cybertron millions of years ago in search of energy, and have finally managed to come home.

It is revealed that the Headmasters are a group of small Cybertronians who departed the planet millions of years ago and crash-landed on the inhospitable planet Master. To survive its harsh climate, a select few Cybertronians constructed larger bodies called "Transtectors", to which they connected as the heads. They have now joined the Autobot-Decepticon conflict. The super-science of the planet Master forms the backbone of the conflict, which consequently became known as the Master Wars.

Source: ANN & TFwiki

Note: Originally meant to be Season 4 of Transformers, Headmasters became an all Japanese production when toy sales declined dramatically in the U.S. Instead, Americans got the 3-part miniseries "Rebirth" to end Transformers in the U.S. The show however, directly continues from Season 3 of American Transformers, assuming "Rebirth" never happened. Every subsequent Transformers anime was produced in Japan.

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