Who is Tom Ditto?

Tom got a note from his girlfriend. It says she hasn't left him, but she's gone. But where the hell is she? When is she coming back? And what is Tom supposed to do in the meantime? With each new clue to Hayley's whereabouts, Tom is forced to question whether he truly knows his girlfriend at all. Because who is Hayley really, anyway? Who is this other strange girl following him? And who, for that matter, is Tom Ditto?