What's the worst thing that you can threaten another person with? Probably it's when you threaten the life of their child. And that's what happens on 'Lie to Me' (Mon., 9PM ET on Fox).

[Spoiler alert.]

Dr. Cal Lightman (Tim Roth) is dealing with the aftermath of a case from the past. Lightman sent a killer to his death for murdering an 8-year-old child. But now, it seems as though the same killer is still on the loose -- which would mean that Cal captured the wrong guy. Did Cal unwittingly prosecute an innocent man?

To make matters worse, it looks like the child-murderer is after Cal's daughter Emily (played by Hayley McFarland). At this point, Dr. Lightman gets a terrifying call over the phone: " ... Do you know where you daughter is?" It might be the worst message that a father could receive.