[s0e13] Planet Of The Dead -

She didn't hesitate. She handed over a necklace of psychically-responsive crystals. The Doctor jammed them into the steering column.

"A trillion people lived here a year ago," the Doctor said softly. "Then something came. Something that eats everything. Not just the people, but the air, the water, the very molecules of the world." [S0E13] Planet of the Dead

"They tear a hole in space, eat the world until it's a desert, and then move to the next one," the Doctor explained, his eyes widening. "And that hole leads straight back to London." She didn't hesitate