Rose likes to shop, so the Doctor takes her to an alien marketplace. After she wanders off, he goes to look for her and runs into someone unexpected who gives him some much needed hope for the future!
The Doctor is a man of many regrets, but there is one person who stands out in his memory as the person he wishes he could see once again. A Time Lock makes this reunion impossible or so the Doctor thought. But even Christmas miracles can have their dangers. (Takes place after The Sacred Flame/The Fires of Karn episodes in Unto the Universe)