Billy: How did I get here?
The Doctor: Same way we did. Touch of an Angel. Same one probably, since you ended up in the same year. No, no, no, don't get up. Time travel without a capsule - nasty. Catch your breath, don't go swimming for half an hour.
Billy: I don't... I can't...
The Doctor: Fascinating race, the Weeping Angels. The only psychopaths in the universe to kill you nicely. No mess, no fuss, they just zap you into the past and let you live to death. Rest of your life, used up, blown away in a blink of an eye. You die in the past and in the present they consume the energy of all the days you might have had. All your stolen moments. They're creatures of the abstract, they live off potential energy.
Billy: What in God's name are you talking about?
Martha: Trust me, just nod when he stops for breath.