This episode marks the first time Doctor Who gets an explicit mention in the series.
The scar on Toshiko's mother's head is the same shape as the boy in the gas mask from The Empty Child (2005)/The Doctor Dances (2005), the story arc which introduced Captain Jack Harkness.
First depiction of UNIT in the series.
This episode takes place in 1899; from December 31, 1999, to January 1, 2000; in 2004; in 2006; in 2007; and in March 2009.
Ianto Jones was born on August 19, 1983.