The Transformers: The Movie

Set in the year 2005, twenty years after the events of the TV series, the film opens as a colossal mechanical planet called Unicron drifts through space, consuming everything in its path — beginning with the inhabited metal world Lithone. Meanwhile, the Decepticons occupy Cybertron while the Autobots operate from two of its moons, planning a mission to resupply from Autobot City on Earth. Decepticon spies intercept Autobot communications; Megatron and his forces hijack the Autobot shuttle en route to Earth, killing crew members Ironhide, Prowl, Ratchet, and Brawn, then use the shuttle as a Trojan horse to launch a surprise assault on Autobot City. The battle is devastating on both sides. Hot Rod and the young Autobot Daniel help hold off the attack long enough for Optimus Prime to arrive. Prime and Megatron fight one-on-one; both are mortally wounded. Before dying, Optimus Prime transfers the Autobot Matrix of Leadership to Ultra Magnus, prophesying that 'one day, an Autobot shall rise from our ranks and use the power of the Matrix to light our darkest hour.' The badly damaged Decepticons retreat aboard Astrotrain; Starscream seizes the moment to eject the mortally injured Megatron and other weakened rivals into space. The floating Megatron is discovered by Unicron, who offers him a rebuilt body in exchange for one service: destroy the Matrix, which is the only force Unicron fears. Megatron accepts, and Unicron reconstructs him as the more powerful Galvatron. Other fallen Decepticons are reborn as Cyclonus, Scourge, and the Sweeps. Galvatron's first act is to return to Cybertron and vaporize Starscream mid-coronation ceremony. He then pursues the surviving Autobots across the galaxy. Hot Rod, Kup, and the Dinobots crash on the hostile planet Quintessa, where the Quintessons — bizarre five-faced beings — capture Hot Rod and Kup and put them on trial with a predetermined death sentence. They escape with help from the Dinobots and a small native creature named Wheelie. Separately, Ultra Magnus, Arcee, Springer, Blurr, and Perceptor crash on the Planet of Junk, where they are attacked by the Junkions, a race of robots who rebuild themselves from wreckage. Galvatron corners Ultra Magnus and demands the Matrix; Magnus attempts to open it as a weapon but it shatters and he is blown apart by Galvatron. Galvatron seizes the Matrix but cannot activate it. Unicron, growing impatient, begins consuming Cybertron's moons. The scattered Autobots reunite on Junk; the Junkions, pacified by a television jingle, become allies and rebuild Ultra Magnus. Both groups fly to Cybertron aboard Junkion ships. In a climactic space battle around the now-transforming Unicron — who converts from planet-mode into an enormous humanoid robot and tears into Cybertron itself — the Autobots breach Unicron's interior. Inside the giant robot, Hot Rod confronts Galvatron. As they struggle, Hot Rod grasps the Matrix; he hears Optimus Prime's voice declare 'Arise, Rodimus Prime,' and he transforms into the larger, more powerful Rodimus Prime. He hurls Galvatron into space and opens the Matrix, releasing an energy wave that destroys Unicron from within. The Autobots escape as Unicron explodes; his severed head remains in orbit around Cybertron. Rodimus Prime leads the Autobots back to their reclaimed homeworld and declares the beginning of a new golden age of peace.

Sources: Wikipedia, Transformers Wiki (tfwiki.net)