Complex sentence: The watch fastens on his wrist and gives him the ability to transform into super-powered aliens which allows him to fight villains while trying to escape the alien warlord Vilgax, who pursues him relentlessly in order to retrieve the Omnitrix.
Split into simpler sentences: The watch fastens on his wrist and gives him the ability to transform into super-powered aliens. He then uses the alien powers and abilities to fight villains while trying to escape the alien warlord Vilgax, who pursues him relentlessly in order to retrieve the Omnitrix.