No one saw a thing

You stand on a crowded tube platform in London. Your two little girls jump on the train ahead of you. As you try to join them, the doors slide shut and the train moves away, leaving you behind. By the time you get to the next stop, you've convinced yourself that everything will be fine. But you soon start to panic, because there aren't two children waiting for you on the platform. There's only one. Has your other daughter got lost? Been taken by a passing stranger? Or perhaps the culprit is closer to home than you think? No one is telling the truth, and the longer the search continues, the harder she will be to find.