Sarah reflexively squeezes his arm as she tries to blink back tears. "Five hundred years? Twenty-four hours? Oh, oh Doctor."

That is an awfully long time. A horrendously long time, even. And more than anything else she's encountered - more than thinking the Doctor had died back on Peladon or facing those horrible spiders - it makes her cry.

Because five hundred years is a long time. And five hundred years means that she probably isn't travelling with him anymore.