Disclaimer: I do not, in any way, own Donna, the Doctor, or the TARDIS.
Warnings: Spoilers for the Series Four Finale. Slightly AU.
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"If she remembers, she'll burn up."
The Doctor sank down beside the control panel in the TARDIS. He was alone now, alone as he hadn't been for many years. What was it like?
Empty. Cold. Silent. The TARDIS, once a haven, now seemed a tomb.
Years passed almost meaninglessly. He still helped out, of course; he was the Doctor. But he never took another human companion.
He was waiting for the day she would remember, so that (he neglected to mention) he would burn too, and be warm again.