The film revolves around the Maximals, led by Lio Convoy; the Predacons, led by Galvatron; and a mysterious device that has crashed on Planet Gaia (a future version of Earth), leading to the arrival of a legendary Maximal warrior, and the protagonist of the original Beast Wars series, Beast Convoy (Optimus Primal). Galvatron also uses the device to summon a monster Decepticon called Majin Zarak, who threatens to destroy all the Maximals.
(Source: Wikipedia, edited)
