Carmen:
"No, but you be careful, because your song is ending, Sir. It's
returning. It's returning through the dark and then Doctor, oh, but
then. He will knock four times."According to Russell T. Davies, 'Planet of the Dead' was the Doctor's last chance to have some fun. It's common knowledge now that the Doctor's just four episodes away from his end, so this was Ten's last chance to embark on one last old-fashioned romp, before the serious business of his dying begins in earnest. Something is returning. Carmen doesn't specify what, but we can be almost certain that whatever it is, it'll mean the end of the road for David Tennant.
