The alien takes the form of a dead loved one, so the colonists call them Ghosts (even though they're corporeal and eat and sleep and have emotions and can even be injured) and Elizabeth's supervisor is to be Dr. Murphy, the man who developed the Ghost Protocol, which states that the Ghosts are to be shunned and ignored completely. Elizabeth and Murphy hit it off immediately, and even start to flirt briefly, before they discover that not only did Elizabeth die in a shuttle crash when arriving on the planet, but she's now Murphy's new Ghost. So they go from sexy banter to complete silent treatment, while Elizabeth has to process her own demise and new and unwanted situation.