There's more to being one of the Doctor's companions than adventures through space and time, according to Doctor Who's Freema Agyeman: The gig is a good resume-builder, too. In an interview with Digital Spy, Agyeman not only explained how Martha Jones got her job with UNIT, but also what to expect when the character goes from her Torchwood appearances back to the Tardis in the upcoming fourth season of Who.
Agyeman explained that the unnamed "highly impeccable source" that helped Martha get the job with UNIT was a very familiar face:
The fact that it was the Doctor? Thing is, when I go back into Doctor Who and the Doctor and Martha meet up again, they do talk about the fact that she's worked at UNIT. So it is re-iterated and explained more. But it was him that got her the job there, yeah... [W]hen they do meet again it's more in a professional capacity. She's outgrown him really. She was always mature, independent and all those things but she was still a bit wide-eyed with him. And that's what's gone. She still really respects him and knows he's far beyond her intellect and the rest of it, but she was dominated by him and now there's a different dynamic there. Ultimately they're the same people, so there's still that connection, but it's a different dynamic.
On that different dynamic:
They've still got this friendship but there's somebody else in Martha's life now so she doesn't need The Doctor in that respect. That part of her soul has been fulfilled. That attraction has dissipated and it's quite professional. It's nice to see how they've moved on, because she did feel quite strongly about him.
And as to the possibility of Martha appearing post-Who season four?
I would like to continue a little bit with Martha. I don't know how, or in what capacity, but I feel there is a little bit more I could do with her. I would like to stay within the family, whichever way... the door is kind of left ajar, in some ways, I guess, so we will see - but I would definitely like to stick around.
Torchwood Interviews: Freema Agyeman [Digital Spy.co.uk]