The Fourth Dimension
The Empty Child
This episode marks the first appearance of former Time Agent and conman Captain Jack Harkness as played by John Barrowman.We don't know the character's real name, as Jack Harkness is an alias he adopted from a doomed World War II pilot. Jack has featured in eight subsequent Doctor who episodes, and starred in two series (so far) of spin-off Torchwood.
Albion Hospital also features in the episode Aliens of London. Scenes at the hospital were filmed at the Cardiff Royal Infirmary.
Are You My Mummy?
The Doctor jokingly repeats the classic catchphrase from this story in Series Four's The Poison Sky, when he puts on a gas mask.
There are conflicting stories regarding the the sound of Dr Constantine's skull cracking when he morphs into a Gas Mask Zombie. It's claimed that the sound was removed before broadcast for being too horrific, being discussed but never added at all, and added especially for the version on the DVD box set.
The gas masks used in the episode were props custom made for the story, and not specifically based on period designs. Normal civilian gas masks from World War II features a single eyepiece, rather than one for each eye.
Children Of Time
Early versions of the script gave it the title An Empty Child - a reference to the first Doctor Who story An Unearthly Child. It also had the working title of World War II at one point.
Captain Jack is a former Time Agent. War criminal Magnus Greel referred to being pursued by Time Agents in The Talons Of Weng Chiang (1977). Doctor Who spin-off Torchwood introduced to another Time Agent, Captain John Hart.