The Doctor and Yvonne Hartman in “Army of Ghosts”

So you find the breach, probe it, the sphere comes through. Six hundred feet above London, bam. It leaves a hole in the fabric of reality. And that hole, you think, "Oh, shall we leave it alone? Shall we back off? Shall we play it safe? Nah," you think, "let's make it bigger!"

It's a massive source of energy. If we can harness that power, we need never depend on the Middle East again. Britain will become truly independent. But you can see for yourself. Next Ghost Shift's in two minutes.

Cancel it.

I don't think so.

[more angry] I'm warning you, cancel it!

Oh, exactly as the legends would have it. The Doctor, lording it over us, assuming alien authority over the Rights of Man!

Let me show you. Sphere comes through. [points the sonic screwdriver at a glass wall, creating a hole with cracks spreading around it] But when it made the hole, it cracked the world around it. The entire surface of this dimension splintered. And that's how the ghosts get through, that's how they get everywhere - they're bleeding through the fault lines! Walking from their world, across the Void, and into yours, with the human race hoping and wishing and helping them along. But too many ghosts, and... [taps the glass, and it shatters]

Well, in that case, we'll have to be more careful. [to her staff] Positions! Ghost Shift in one minute.

Miss Hartman, I am asking you, please don't do it.

We have done this a thousand times.

THEN STOP AT A THOUSAND!