A Mysterious Disappearance

Missing: one frightened little girl. Name: Bettina Miller. Description: six years of age, average height and build, light brown hair, quite pretty. Last seen being tucked in bed by her mother a few hours ago. Last heard: 'ay, there's the rub,' as Hamlet put it. For Bettina Miller can be heard quite clearly, despite the rather curious fact that she can't be seen at all. Present location? Let's say for the moment... in the Twilight Zone.