They haven't slept for three days.
For the Doctor this is nothing, a tiny itch at the back of his mind that suggests he get some rest sometime soon, easily ignored.
For Martha he can tell it's becoming a problem, a thick ache of exhaustion weighing down her entire body. He notices her drooping eyelids, her dulled reflexes, her faltering steps.
He tells her to run.
It's sharp and it's brutal like every breath she's drawing through over-worked lungs. The Doctor grabs her wrist when she stumbles, her arm when she falls behind. He wants to see how far she will go, how far he can take her under this guise of adventure.
When they get back to the TARDIS, Martha mentions she's a little tired. The words are cautious, almost bitter. The Doctor knows she is measuring herself against a ghost whose presence is powerful enough to legitimize the possibility of it having possessed super powers, such as a lack of need for sleep. The Doctor knows he should tell her to go to bed, to promise breakfast followed by a trip to a nice clean planet, possibly with shops, whenever she's ready.
Instead he asks her if she's ever seen an ice storm in the middle of an ocean. They don't do much running then, but they do get awfully wet.
There are always voices in the Doctor's head, centuries and galaxies and lifetimes of voices, but there are two in particular that make themselves heard as he battles a sail in the freezing rain. One, the loudest because blimey she icould/i shout, tells him he's being a reckless idiot and sounds very much as though the speaker would like to slap him hard enough to put a crick in his neck. The other is softer, sadder, and the reason he doesn't want to sleep or stop or breathe.
This other voice doesn't say much, mostly just his name.
It filled his mind after the terrible day he stood in front of a blank wall that meant the universe. He let it guide him in his manic search for a way, any way, through, because a blank wall was not a goodbye and he couldn't let her go, not without even a goodbye.
Then Donna came and she was louder than the voice and the Doctor let her be, let her ring in his ears, pulling him back from the brink that promised eternal silence. He made a promise to her too, but the words were lost while the voice returned, with no Donna and no goodbye to hold it back.
The Doctor went to sleep then, the deathlike sleep that normally only came to Time Lords when accompanying a mortal wound. The voice was the soundtrack to every single one of his dreams.
When he awoke, he didn't feel better. He just felt emptier. The voice reverberated in the emptiness inside of him, and he sought to chase it with other sounds, to make such a din that picking out the voice would be a deliberate choice.
He didn't want to lose it. He just wanted it blunted, weakened. Deadened.
He stopped sleeping, he started moving, and he met Martha Jones. She is smart, strong, and capable. She believes the Doctor to be the same, because instead of showing her the emptiness inside he had shown her a mirror. Her eager, untempered vitality is excellent for absorbing echoes of the past, and he needs it trapped and held within him.
The Doctor knows he should be kinder to Martha, knows he should ask about her family, hear the stories behind her tattoos, learn the name of her favorite song. He knows she fancies him, he knows she believes in him, he knows she wants him to reciprocate both feelings and is holding on to quite a lot of hope that he will. He knows so many millions of facts and truths about the universe, but not one that will embody the voice in his head and bring a smiling, warm-hearted blonde back to his side.
He doesn't know how to tell Martha he's all out of love, all out of faith. He doesn't know how to tell her he's a hollow shell, only capable of giving her shallow reflections of what she gives him first. He doesn't know how to tell her she's better off without him, her seeing the universe has a price and he's collecting the interest, she deserves more.
He doesn't know how to tell her goodbye.
So he pushes her instead, harder and harder, wanting to see if she'll say it first, if she'll break like he's broken, if she'll cut through to the emptiness and be swallowed by it.
They haven't slept for five days.
For the Doctor this is nothing, a persistent itch at the back of his mind that says he needs to rest soon, determinedly ignored.
For Martha it's more than her body can physically take, and she collapses in the console room of the TARDIS. The Doctor feels cold and vile as he looks at her lying on the floor, part of his brain remembering another female sprawled on the very same grating but very much awake, part of his brain remembering darkened rooms and masked guards, part of his brain thinking of the way Martha smiled at him while trapped on the moon being menaced by police that resembled rhinoceroses and killer alien leeches that resembled little old ladies.
In her last remaining moments of consciousness, Martha reaches out to him, and he reaches back. Their fingertips meet, and she asks him to wait, as though in a few moments she'll ready to go once more.
But in her near-delirious stupor, the actual words she uses are hold on.
The emptiness inside the Doctor rings with a piercing note, like the harmonic of a string cut in half. As he lifts Martha in his arms and carries her to her bedroom— which the TARDIS shoved close to the front days ago— he is aware she is no longer absorbing the echo of the voice, but adding to it.
The voice is loud as he pauses in the silence of Martha's room, as if it recognizes this new space has no old memories and is rushing to fill the void like matter into a vacuum. The Doctor lets its sound fill his head as he monitors Martha's breathing, and eventually the two noises form a sort of rhythm, beating like the two hearts he's been trying so hard not to feel inside his chest.
He answers them both, Martha's murmured plea as well as the voice, with the only words he ever uses in response to the sad, broken sound.