A very good episode with lots of humour and eeriness. The Doctor and Rose follow a time machine back to London during WWII. There they meet Captain Jack Harkness, the spooky, empty child of the title, searching for its mummy and a hospital full of patients with symptoms just like that of the empty child.
Lots of good stuff in this episode. The Doctor decides to look for alien technology on foot. Rose wants him to do a scan. The Doctor, who did do this in the past (as the Fifth Doctor), doesn't want to. Rose is then delighted when Jack does.