Disclaimer: I don't own Doctor Who.
She enjoys this more than she ought to. The flesh-eating mist might be gaining on them, but Martha doesn't care. The Doctor's hand is sweaty in hers, their fingers sliding against the other's. He's pulling her along, though he can't know where they are, where they're going, any more than she does.
Their feet thunder on the springy ground, a duet of muffled sound. The mist glides on somewhere behind them, moving fast but completely silent, how far or near she doesn't know.
It's all very surreal, even more so than catpeople, Daleks, Shakespeare. The Doctor is her fixed point right now, and in the past, and in the present, and in anyplace he takes her, because she would be lost without him. She wishes he would feel the same way about her, and sometimes she thinks he does, if only as a warm body to hug, as someone to look after, someone to explain things to, someone to share a cuppa with in the mornings. She glimpses a flash of his red trainers in the twilight.
She's running for her life, but she's free, she's happy, she's in love. On this nameless world she's not the responsible sister, the good daughter, the overwrought medical student. Here, she's only running, and as long as there's a flesh-eating cloud behind her, she can convince even herself that's what she's running from. A couple of trees flicker by.
She stumbles and he grabs her hand tighter, anchoring her. (Further. Again.)
The Doctor risks looking over his shoulder and stumbles immediately; she manages to steady him with her weight. "It's receding!" he shouts, the words ringing out across the plains.
They keep running, though, at the same pace as before. Martha is exhausted, her lungs burn, her legs have gone numb, her heels are killing her, and yet she doesn't ever want to stop running. She shrieks, because it's exhilarating, it is, running like this through a dingy old moor who knows where and when, outrunning something she saw dissolve a horde of cow-like somethings in the blink of an eye. The Doctor shouts something unintelligible and punches the air with his free hand.
The mist is in front of them. Just like that.
"Unfair!" the Doctor chokes out.
Martha tightens her grip on his hand.
They turn as one and run back the way them came. Their fingers chafe against the other's, her hair is in her mouth, she can scarcely draw breath and that mist was a little tougher than she'd expected, but she doesn't care.
She enjoys this more than she ought to.