"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 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. Marvel's promotional tagline for the event was "Who do you trust?"