Twice In Every Lifetime...
This is Christopher Ryan's second role in Doctor Who. He previously played Lord Kiv in Parts 5-8 of The Trial of the Time Lord (1986). Chis is probably best known for his comedic roles, including Dave Hedgehog in Bottom and Mike in The Young Ones.
UNIT: The Early Years
UNIT first appeared in 1968's The Invasion, in which the military organisation - with the help of the Second Doctor - fought the Cybermen.
UNIT has now been rebranded as the Unified Intelligence Taskforce, having previously been known as the United Nations Intelligence Taskforce. This followed a request by the United Nations shortly after the series returned to TV in 2005.
Sign Of The Times
When communicating with each other, UNIT use call signs first used in the Classic series. These are mostly based around greyhound racing. Personnel are usually designated a Greyhound call sign, whereas a base of operations is known as a Trap.
UNIT Dating, Part One
The Doctor's reference to working for UNIT "Back in the 70s - or was it the 80s" refers to inconsistencies in the dates for UNIT stories in the Classic series. It was generally believed that UNIT stories were set slightly ahead of the present day, confirmed by Sarah Jane Smith's claim she was from 1980 in 1975's Pyramids Of Mars. 1983's Mawdryn Undead confused things somewhat, by featuring a retired Brigadier Lethbridge Stewart (the former head of UNIT's UK operation) in 1977!
UNIT Dating, Part Two
Martha's fiancé Tom Milligan is the doctor from Last of the Time Lords.
The Sontaran Stratagem is the fifth story to feature the clone warriors. They have previously appeared in The Time Warrior (1973-4), The Sontaran Experiment (1975), The Invasion Of Time (1979) and The Two Doctors (1985).
The Sontarans' weak point is their Probic Vent. It's a small connector at the back of their necks that allows them to plug into an energy source to recharge. Hit it with something heavy or stick an arrow (The Time Warrior (1974), knife (The Invasion Of TIme (1978)) etc. into it and they're in big trouble. Sontarans see the Probic Vent as an advantage, however. It means they must always face their enemies in battle.
The Twin Dilemma?
Despite being a clone species, no two Sontarans appearing in Doctor Who have ever looked identical! We're not counting the reuse of the costumes from 1985's The Two Doctors in the same year's Jim'll Fix It mini adventure, A Fix With Sontarans, though.
Sowing The Seeds...
Did you notice that the idea of Atmos has been around since the start of this series? Take a look a look at the windscreen of the taxi that arrived to pick up the doomed Stacey in that episode.
Compiled with the help of Peter Ware and Helen Raynor.