Complex sentence: The story involves a subversive, long-term invasion of Earth by the Skrulls, a group of alien shapeshifters who have secretly replaced many superheroes in the Marvel Universe with impostors over a period of years, prior to the overt invasion.
Split into simpler sentences: The story involves a subversive, long-term invasion of Earth by the alien Skrulls. Capable of shapeshifting, the Skrulls have secretly replaced many of Marvel's heroes with impostors over a period of years, prior to the overt invasion.