The ShapeShifter is an alien nano-virus that is trying to take over the Crew and escape from the Pandora Celeste.
 
Each player is dealt 2 cards at the start of the game.  One of these may be the ShapeShifter.  Every time you are alone with a Nasty or other member of the Crew, you must swap one of your 2 cards.  Will you infect another victim, or will you be the one who is infected?
 
But if you're the ShapeShifter, watch out!  Because if your 'victim' resists your infestation, you are exposed and you become the Hunted...