[It's a relief to finally receive her answer; despite the fact that he trusted Rose to take care of herself, there'd still been a gnawing worry within him. He's almost able to keep the relief out of his face when he responds, although she might be able to catch it if she looks closely.]

Someone's just arrived here in the city- someone very dangerous. An old- [He misses a beat here, because- the Doctor didn't quite consider the Master an enemy, he knows, but to Jack, that's all the other Time Lord is. An enemy. So he sticks with that, nevermind the Doctor's emotional attachment.] enemy of the Doctor's. Another Time Lord, one who survived the war. He goes by 'the Master.'