On the Decepticon spaceship, retreating from the battle at Autobot City, the Decepticons are reinforcing their general antagonist-ness by infighting over who gets to lead the Decepticons after Megatron's fall (Megatron isn't actually dead by this point). This leads to the lot of them fighting inside the ship (another Decepticon -- scale isn't closely adhered to in the animated versions of the show), which then lurches around a bit comically.
Megatron and the other nearly-dead Decepticons are thrown out of the spaceship by the healthier decepticons. They drift through space until they meet (plot twist) Unicron. Unicron has something approximating a philosophical dicussion (to eight year-olds) with Megatron, then builds him into a new body as Galvatron (with Leonard Nimoy's voiceover). The other fallen Decepticons are rebuilt into assorted new characters. All the new characters are indicative of the direction things took after this point in the Transformers lifespan -- the transition from cars and trucks to spaceships and other more abstract manifestations. Things had definitely changed.
Galvatron and his minions fly to Cybertron, where Starscream is being crowned the new leader of the Decepticons. He then kills Starscream at the coronation (held in a sort of roofless hall with statues of famous Decepticons along the sides), and resumes leadership of the Decepticons surviving this much of the movie.
Killing off Starscream, a longtime antagonist, probably didn't cause too many tears amongst the eight year-olds. Starscream the toy had die-cast steel components and quite a few metal pins; the cockpit canopy came off fairly easily, which may have spelled 'choking hazard' to execs trying to broaden the age-range to which the toys could be marketed. The replacements for the jet characters were almost entirely plastic.
Getting him fairly well stuck back into the foreground of the movie, Unicron then eats two moons of Cybertron -- the ones with Autobot bases on them. Things are supposed to look Very Bad for the Autobots right about now. In doing so, Unicron also eats several more old Autobot characters, only one of which will reappear later on.
Here's Prowl (the Autobot policecar) fleeing one of the moon bases shortly before Unicron eats him. The main thing to note (other than that Prowl disappears from the movie at this stage) is the nice reflective thing on the floor, which never occurred in the TV series, presumably because it costs too much.less, more, almost none