Summary: Martha Jones always knocks.
Spoilers: Human Nature
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Martha Jones always knocks.
The Doctor does not notice this at first; there aren't that many doors in the TARDIS after all, and to be perfectly honest, in those first few weeks after their extended sojourn in 1913 he has more important things to worry about than his companion's domestic habits.
Stil, it strikes him as a bit odd when she softly knocks on the kitchen door in the morning and then waits for his puzzled call to enter before sticking her head round the door to sleepily enquire after a cup of coffee.
Now, the Doctor cannot be too sure about this – it's rather rare that he shares meal times with Martha – but he fuzzily recollects her simply breezing through the kitchen door as if she owned the place, making a beeline for the fridge.
That, however, was before – and before is such a long time and a lifetime ago.
Sitting in the library one night, the Doctor's reading is interrupted by a light rapping on the door. He merely grumbles out half an acknowledgement while pushing his glasses up his nose, turning a page shortly thereafter. He doesn't know how much time has passed before he looks up, frowning. There's something slightly wrong, a small missing link in the chain of events.
He only remembers the next morning, but Martha doesn't mention it and neither does he.
It's only when he absent-mindedly asks her one day – "What's with the knocking, some sort of 21st century custom I wasn't aware of, announcing your presence in your own home? Though to be fair, there are some species that…" – and she neither snaps back a witty reply nor blushes in pleasure at him calling the TARDIS her home that he begins to have an inkling that something might be not quite right. Because there she is, standing in the doorway and looking utterly miserable for a moment before shrugging slightly and flashing him a bright smile.
They don't talk about it; of course they don't.
The Doctor cannot undo the past – well, not like that, anyway. He can't apologise to Martha either. Firstly because he hasn't the foggiest idea what exactly to apologise for and secondly because he just knows that she would simply brush it off – or, worse, take it in the entirely wrong way. What he can do, though, is make a request to the TARDIS.
When Martha enters the kitchen the next morning she mumbles a greeting, then pauses, staring at the open arch where the door used to be.
Grinning at his companion, the Doctor licks some jam off his toast. He knows in that moment that he managed to get things right, for once; her mildly bewildered smile is proof of that.
"Door's always open to you, you know."