The Zygon Inversion: The Fact File

Kate reveals that she dealt with the Zygon in T or C with ‘Five rounds rapid!’ The line was first used by her father, Brigadier Lethbridge-Stewart, in The Daemons. In that story the Brig instructs a UNIT solider to open fire on Bok, specifying, ‘Chap with wings, there: five rounds rapid!’ The expression ‘Five rounds rapid!’ became so identified with Lethbridge-Stewart that the actor who played him, Nicholas Courtney, took it as the title for his 1998 autobiography.

The Mire helmet on display in the Black Archive

Incidentally, that same adventure –The Daemons - starred Alec Linstead as UNIT’s Sgt. Osgood…

When the Doctor says, ‘…and I mean that most sincerely!’ he’s mimicking Hughie Green (1920 - 1997), an English television presenter best-known for hosting Opportunity Knocks. His catchphrase was, ‘And I mean that most sincerely, folks!’ usually delivered in a broad American/Canadian accent.

The Black Archive was introduced in The Enemy of the Bane, the final story of The Sarah Jane Adventures’ second season. It first appeared in Doctor Who just under 5 years later, during 2013’s The Day of the Doctor.

The Black Archive looks similar to when we last saw it during the fiftieth anniversary special although one obvious addition is the helmet of a Mire, prominently displayed behind glass. The Mire, of course, were the alien warrior race defeated by the Doctor in The Girl Who Died.

‘Let me ask you a question about this brave new world of yours…’ The Doctor is quoting Shakespeare with the ‘brave new world’ reference. The expression was introduced by the Bard in his final play, The Tempest. The Doctor has used the phrase before – appropriately enough, in The Shakespeare Code.

Harry Sullivan in Terror of the Zygons. The Zygons were originally created by Robert Banks-Stewart.

Z67 is named ‘Sullivan’s Gas’. This suggests a link to the Fourth Doctor’s companion, Harry Sullivan… ‘One of our staff was a naval surgeon…’ Kate stated in The Zygon Invasion, discussing the UNIT member who developed Z67, adding that the medic, ‘Worked at Porton Down…’ This further strengthens the Harry Sullivan connection as he was a naval surgeon and in Mawdryn Undead, the Brigadier told the Doctor that Harry had been seconded to NATO and was doing something ‘very hush hush’ at Porton Down.

When the Doctor expresses surprise that Osgood doesn’t know what TARDIS stands for, she replies, ‘I’ve heard a couple of different versions.’ This could be a sneaky reference to the fact that the Doctor himself has given slightly differing versions of the acronym… In An Unearthly Child, his grand-daughter claimed she made up the name from ‘time and relative dimension in space’, a declaration he didn’t correct. But later he began telling people that the D stood for dimensions. When the Doctor explained the ship’s name to Rose Tyler in her debut adventure he reverted to the ‘time and relative dimension in space’ version!

‘There was another box… I was going to press another button.’ The Doctor is referring to events covered in The Day of the Doctor when he seemed set upon activating the Moment, an enormously powerful weapon which at certain points resembled a box, similar in appearance to the Osgood Box.

The Moment from The Day of the Doctor

At the end of the episode we again see Osgood wearing what appears to be a tank top modelled on the one favoured by the Seventh Doctor. She’s also wearing a brown duffle coat, very similar to one worn by the Time Lord in The Curse of Fenric.