"The Unicorn and the Wasp" is a step back from some of the darker themes of the early part of Season Four, bringing out the lighter side to life with the Doctor, while still indulging in the expected amounts of jeopardy, death and monstrousness. It's close in spirit to the two other new series stories in which the Doctor meets a famous author from the past, with lots of in-jokes and references for fans of the writer in question.

But as this is Doctor Who, it also uses his time travel to offer a solution to a famous genuine historical mystery, using wasps.

Here are a few things that you should keep an eye out for, the next time you watch.

(The episode is available on Amazon and iTunes.)

This is yet another new Doctor Who Season Four adventure to benefit from the presence of an actor best known to British viewers from their part in an iconic TV comedy. Felicity Kendall was the star of the 1970s eco-comedy The Good Life, a show that has been repeated often and is very highly regarded. She is, therefore considered to be something of a national treasure:

https://www.youtube.com/watch?v=vdF-64yD-8U

The story was originally filmed with different beginning and end, featuring an aged Agatha Christie whose memory has started to return in the form of flashbacks and dreams. The Doctor and Donna are seen to visit her, and show her the cover of Death in the Clouds with the huge wasp on the cover (a Fontana paperback print from 1957 that genuinely exists) from the year 5 billion.

Another abandoned script idea was that the Doctor would drive the car into the Vespiform, forcing it into the lake at the very end of the story. David Tennant requested that the ending be altered for fear that the Doctor appear to be a heartless murderer.

When the Doctor says he met Charlemagne, this isn't a throwback to a classic adventure, or an old Doctor Who novel, it's a direct reference to the short story "The Lonely Computer," written by Rupert Laight, that appeared on the Doctor Who website in 2008, the same year this episode was broadcast. In the story—which is still available to read here—the Tenth Doctor and Donna do indeed meet the French king and Emperor of Rome.

As well as references to Agatha Christie's detectives Miss Marple and Hercule Poirot, the characters in the dinner party have been arranged into characters that broadly follow those in the board game Clue (known in Britain as Cluedo). So as well as the lead pipe, revolver, knife, candlestick and length of rope (well, curtain cord) used as murder weapons in the game, there's Professor Peach representing Professor Plum, Reverend Golightly for Reverend Green, Clemency Eddison is Mrs Peacock, Robina Redmond is Miss Scarlet, Colonel Curshiby is Colonel Mustard and Miss Chandrakala is Miss White. Oh and there's a death in the library.

Keep your eyes on the serving staff, as one of the footmen is played by Sandy McDonald, David Tennant's dad, who was visiting him on set and offered the role on the spot.

Donna makes a sardonic reference to "the planet Zog" as a generic example of the sort of name planets are often called in science fiction. But there actually is a planet of that name in the Whoniverse. It's the location for the Zaggit Zagoo bar in which Captain Jack Harkness meets Alonso Frame in "The End of Time". The same bar also appears in the story "The Red Bicycle," part of The Twelve Doctors of Christmas short story collection and the planet forms the backdrop for the Captain Jack comic adventure "Overture," from Torchwood: The Official Magazine.

There's a really subtle moment concerning British slang that is easily missed. As the Doctor first realises he's been poisoned, he rounds on Davenport and gasps "ginger beer!", only to met with the tetchy retort, "I beg your pardon?!" as if Davenport has been personally affronted. This might be because in Cockney rhyming slang, "ginger beer" is a term used to denote homosexuality, to rhyme with "queer".

While we're on etymology, the Doctor makes a little pun to himself, using the term "buzzed off"—from "buzz off" meaning "go away"—when the wasp that first terrifies Donna has flown out of the window. That expression will have been quite new in 1926, as it came from the early days of telephone technology, in which buzzers were used to alert people of incoming calls.

The script is riddled with punning references to Agatha Christie's book and short story titles, something Russell T Davies admitted he rather enjoyed doing. These include:

• • Donna referencing Murder on the Orient Express and Miss Marple.
  • Agatha saying "sparkling cynanide" when the Doctor is poisoned (a future novel title).
  • The Doctor's choice of clothing makes him The Man in the Brown Suit.
  • The Colonel lying about his ability to walk and Robina Redmond being the Unicorn are both nods to After the Funeral.
  • Miss Chandrakala's describes the Professor's book as a "dead man's folly", another novel title.
  • Donna refers to Professor Peach as The Body in the Library, while Hart says the murder has put the Cat Among the Pigeons.

https://www.youtube.com/watch?v=EjHTT974G6U

• Agatha refers to the murderer as Nemesis and The Secret Adversary.
• The Doctor checks the spelling on the piece of paper Agatha found, N or M?
• The last thing the Professor says is "Why didn't they ask... heavens!", a nod to Why Didn't They Ask Evans?
• Agatha believes the giant wasp is a trick, noting that They Do it with Mirrors.
• Lady Eddison says Chandrakala had An Appointment with Death and that Christopher was "taken with the flood," a nod to Taken at the Flood.
• The Colonel asked Agatha to reveal what she knows, to put her Cards on the Table.
• The dinner table is decorated with a vase of Yellow Iris.
• Agatha describes four of the residents of the property as living in a Crooked House, while the Doctor describes the evening as an Endless Night.
• The Doctor refers directly to The Moving Finger.
• Once Reverend Golightly has died, Agatha notes that Death Comes at the End.

And finally, the Doctor sums up the story as "murder at the Vicar's rage," after Murder at the Vicarage, raising Donna's eyebrows at the rotten pun.

NEXT: 10 Things You May Not Know About ‘Silence in the Library’

Now read the rest of the 10 Things You May Not Know About Doctor Who archive.