The defense condition (DEFCON) is a measure of the activation and readiness level of the United States Armed Forces - although NATO appear to have adopted it, too, in the Doctor Who universe. DEFCON 1 refers to maximum readiness. It is not certain whether this has ever been used, but it is reserved for imminent or ongoing attack on US military forces or US territory by a foreign military power.
Rattigan designates Castor 36 as Earth-point-2, rather than the more conventional 2.0. It's similar to the way as Cybermen we referred to as humans-point-2 in 2006's The Age Of Steel.
Operation Golden Age
In 1974's Invasion Of The Dinosaurs, another group of humans believed they were going to start a new life on a new planet. Sadly the truth was that evil politicians and scientists planned to roll back time on Earth, deleting all of Human history.
When the Doctor cut's off Staal's speech on the video screen in mid flow, a clip from CBeebies cartoon Tommy Zoom is featured. The original plan to use a clip from Shaun The Sheep fell through.
Green Issues - Part One
In the script, the Valiant is instructed to have the same green lasers as the Torchwood ones used to fire at the Sycorax in the The Christmas Invasion (2005).
Green Issues - Part Two
Mention is made of the Sontarans' ongoing war with the Rutans. Blobby green jellyfish from Ruta 3, the Rutans have the ability to shape-shift and impersonate creatures they have killed. The only story to actually feature a Rutan was 1977's Horror Of Fang Rock.
Speel Or No Speel?
We wonder if a speelfox is related to the speelsnape that the Sixth Doctor mentions in 1985's Revelation Of The Daleks?
It's A Puppet!
Catherine Tate claims that she originally thought all the Sontarans were animatronic, rather than played by actors.
The Doctor and Mace namecheck the Classic Series' head of UNIT's UK branch, Brigadier Lethbridge-Stewart. Sadly the Brig didn't make it out of Peru to join in the action.
Compiled with the help of Peter Ware.