This is how Megatron returns goes in Darkness Rising Part 1.
[Soundwave plays a recording]
Starscream: Those transmissions are emanating from deep space. I would hate to waste the energon unless you are absolutely certain.
Starscream: Then lock on transmission coordinates and activate the space bridge.
[Soundwave does so]
[In space, the Spacebridge opens and a fighter jet zooms out. It turns into Megatron]
Megatron: Decepticons! I have returned.
[To be continued...]