Welcome aboard the Constellation Princess, where the last survivors of an ill-fated interstellar cruise are attempting to abandon ship.  The journey through its bowels is dangerous as this luxury liner is immense, and there's a hostile shape-shifting alien aboard, waiting to strike from the shadows.  But there's something even more troubling: the voice of a mysterious passenger calling out over the ship's radio, warning the survivors to stay clear of the path to the lifeboats.