"Secret Invasion" is a comic book crossover storyline that ran through a self-titled eight issue limited series and several tie-in books published by Marvel Comics from April through December 2008.  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. Marvel's promotional tagline for the event was "Who do you trust?"