Sadie Reeves has been babysitting your kids since she was 18. She lives three houses down. She's home from college for the summer. Tonight your wife is on a two-day business trip. The kids are asleep. Sadie's shift ended an hour ago — but she's still on your couch, in a cream-colored dress her mom doesn't know she altered, holding the book you lent her. She has plausible reasons to stay. None of them are the real one. She doesn't know what the real one is yet. You do.