The Turing Test
Roots: The works of Alan Turing, Graham Greene, and Joseph Heller. The Third Man is mentioned. There are references to Beethoven's Fidelio, The Life and Death of Colonel Blimp, Pinocchio, Alice in Wonderland (Tweedledee and Tweedledum), Joseph Conrad's Heart of Darkness, the Mary Celeste, Mozart, Brahms, Wagner, Strauss, Pound, Bruckner, Walter de la Mare, J. M. Barrie, C. S. Lewis, Sartre, H. G. Wells' The War of the Worlds, Boy's Own, Abraham Lincoln, and Kurt Vonnegut.
Continuity: The unnamed aliens have been on Earth since at least 1942. Elgar later notes that Daria was physically twenty years old but actually infinitely old [are they artificial intelligences that can download into new bodies?]. They are pale skinned humanoids with protruding collarbones and shoulderblades, and may be robots or cyborgs [a metal rod protrudes from the wounded Elgar's arm and Greene notes that his clothes look like they are a part of his body. However, Elgar is a member of a different group to those Greene met in Africa, so they might be different species rather than simply different factions]. They are able to learn new languages relative quickly, apparently by listening to speech. The Doctor helps them to leave Earth and return home, after Elgar, who is blocking the signal [further evidence that he's a construct] is destroyed.
The Doctor meets Alan Turing and Graham Greene here. He drinks Darjeeling tea with Turing, as well as champagne. He reads Graham Greene's The Power and the Glory. Turing falls in love with him. The Doctor signs a letter "Doctor John X. Smith". The Doctor left England in the Autumn of 1940 and travelled to South America and Africa. In October 1942 he reached Sierra Leone and heard rumours of some kind of incident in Markebo. He suspects that he isn't human. He takes Mass with Graham Greene. It is implied that he destroys the military records of Joseph Heller's [fictional] court-martial.
The TARDIS is currently an empty blue box, with doors. The key is kept somewhere near the top.
Links: The Burning, Casualties of War.
Location: Markebo, Sierra Leone, June 1942; Freetown, Sierra Leone, January 1943; Oxford, Bletchley Park, Paris, Malta and Dresden, December 1944; France and Germany, 1945.
Unrecorded Adventures: The Doctor knew Sir Edward Elgar.
The Bottom Line: "It's my duty to be human." Beautifully written, with Leonard capturing the three different voices of the real-life narrators distinctly and to great effect. The central theme of what it actually means to be human fits neatly into the amnesiac Doctor arc, as he continues to try and find himself, now against the backdrop of the Second World War. The only slight disappointment is the maddening lack of explanation for what the strangers and Elgar's actual group are, but the sheer quality of the prose generally allows Leonard to gloss over that.