In the original
Transformers series (i.e. not
Beast Wars), Megatron was the leader of the
Decepticons. He transformed from giant robot to a smaller-sized
Walther P-38 hand gun.
In
robot mode, Megatron has a giant cannon on his right arm, which would fire a deadly
blast. But he was just as deadly when transformed into a gun; many times, Megatron would transform into a gun, and another
Decepticon (usually Starscream) would fire him.
Ruthless and cunning, Megatron was sometimes referred to as "The Slag Maker" because of his destructive abilities. Megatron is also a military
genius. Very few can match Megatron's physical
strength and intelligence; one of the few equals of Megatron is
Autobot leader
Optimus Prime.
In the series, Megatron wanted to comquer the planet
Cybertron, and
destroy any
Autobot who stood in his way. He also wanted to tap the planet
earth for all its natural energy resources. While he has come close, Megatron has never been victorious in either.
In
Transformers the Movie, Megatron and Optimus Prime fought to the
death. While some would argue that Megatron won that battle, it was only because he was using Autobot
Hot Rod as a shield, so that Optimus wouldn't fire back. Later, Starscream jettisoned Megatron off of their escape shuttle, so that he could become
Decepticon leader.
But Megatron, who still had life in him, was re-built by the giant transforming planet
Unicron into a meaner, crazier robot named
Galvatron. Galvatron would kill Starscream and become Decepticon leader again.
Note: In the Transformers Generation 2 toy line and comic book, Megatron was rebuilt by
Cobra (the terrorist organization in the
GI Joe series). Megatron's new transformation was into a green and purple military tank. He did not appear like this in the Transformers Generation 2 cartoon, since it was just the old cartoons with a new beginning title sequence.
However, this tank-Megatron appears in the Japanese Beast Wars II cartoon as the
Destron Megastorm--Galvatron's little brother. (It's true; ask
robotech_master if you don't believe me)