Never played Halo, but it looks a bit like it, yes. I'm guessing the key is some genetic code to humanity or something, but that'd make sense.

Of course, the Donna Noble = Domina Nobilis = Roman Time Lord = Romana speculation might be a different answer, but that'd be too silly, even for Rusty. Actually, no it wouldn't, he'd love to pull that sort of stupid.

Jack Harkness is not all that

True, but I probably still would.