When David (Mark Ruffalo) sublet his quaint San Francisco apartment, the last thing he expected--or wante--was a roommate. He had only begun to make a complete mess of the place when a pretty young woman named Elizabeth (Reese Witherspoon) suddenly shows up, adamantly insisting the apartment is hers. David assumes there's been a giant misunderstanding... until Elizabeth disappears as mysteriously as she appeared. Changing the locks does nothing to deter Elizabeth, who begins to appear and disappear at will - mostly to rebuke David for his personal living habits in her apartment. Convinced that she is a ghost, David tries to help Elizabeth cross over to the "other side." But while Elizabeth has discovered she does have a distinctly ethereal quality - she can walk through walls - she is equally convinced that she is somehow still alive and isn't crossing over anywhere.

As Elizabeth and David search for the truth about who Elizabeth is and how she came to be in her present state, their relationship deepens into love. Unfortunately, they have very little time before their prospects for a future together permanently fade away.