Roots: The Cadfael novels, The Name of the Rose. There are references to Aristotle, Hobbes, Shakespeare, Sophocles, Brunel, Boethius, Pythagoras, Plato, Grosseteste, Augustine, Avicenna, Ptolemy, Euclid, Averroes, al-Kindi, Pliny, al-Farabi, Copernicus, Galileo, Leonardo da Vinci, and Newton. The destruction of the Great Library at Alexandria is mentioned. Darvill-Evan's excellent essay at the end of the novel explains that Bacon was real, although little is actually known about him.
Continuity: The alien race is not named. They are apparently other-dimensional and travel to earth using a module that can traverse dimensions, apparently by traveling through a null dimension. They are non-corporeal and normally inhabit the bodies of creatures called Ikshars when they wish to adopt a physical existence. They can inhabit and influence humans. They can travel through time and change history in the process. They note that when in a human or Ikshar host, they are forced to exist monochronously, suggesting that they do not perceive time in a linear fashion. They refer to the Nargrabine Military Council allowing them to travel only when the mechanism allowing precise manoeuvring of their module was disabled, and they also discuss having fought in the face of Nargrabine aggression they may have been exiled by the Nargrab who may either be another race or a faction of their own race. Transferring between physical hosts requires an energy source. The aliens seem ignorant of many aspects of biology and chemistry, easily believing that Bacon can create an Elixir vitae that will cure plague. The alien that possess Thomas's mind develops a taste for killing, although it is possible that it gains this from Thomas rather than of its own accord. When Nyssa stabs him in the head, it becomes trapped in the metal splinter from the blade.
Time Lords can recall the events of altered histories before they were changed. The Doctor meets Nyssa after she has left him, but before he meets in The Keeper of Traken and discovers that she will travel with him. He is careful to avoid learning details of his future, but nonetheless must therefore pretend not to know her when he meets her in that story. He is traveling alone [therefore setting this story during the period that he first encounters Xoanon (The Face of Evil), between The Deadly Assassin and The Face of Evil, or between The Invasion of Time and The Ribos Operation prior to his completion of K9 Mark II. Given the tone of this story I set it between The Deadly Assassin and The Face of Evil]. The Doctor tells Alfric that in his order it is customary to pray in private [he's presumably talking about the Time Lords, although they probably don't actually pray and he tells Alfric this to avoid joining him in prayer]. He further tells him that his order is a contemplative house called the Prydonians and swears on the sash of Rassilon (the holiest of the relics preserved by my order) (The Deadly Assassin). He has never met Boethius. He remains unaware of the alien involvement in the change to the timeline.
Nyssa traveled with the Doctor for two years. Her Home is maintained by an artificial intelligence (itself called Home), which has multi-billion-synapse organic circuitry. Her cellular structure has been safeguarded against degeneration. She conquered Lazars Disease (Terminus) and traveled for six years, turning her hand to microbiology, diplomacy and fund-raising as circumstances dictated. She joined relief workers on Exanos, airlifting food to Parety a town surrounded by warlords engaged in a vicious civil war; as she departed the planet, one of the warlords detonated a nuclear bomb, which triggered a chain-reaction of response across the planet, engulfing the planets surface in a nuclear holocaust. She was the last field nurse to leave the hospital on Brallis, where she had to cut open the pustules of patients whilst her legs were infected by leech-grubs, which writhed beneath her skin. By 3488 she was thoroughly exhausted by all the horror she had seen and took a job as a lecturer in technography at a university. She is writing a thesis on the origin of science on Earth, specifically on tenth century scholar Roger Bacon. The aliens' intervention in Bacons life changes history, and therefore her thesis, which instead concerns Isambard Kingdom Brunel. Her family on Traken had a crypt for family members. Emotionally drained by the events she has seen since leaving Terminus, she stows away in the TARDIS seeking simpler times in thirteenth century Oxford and is so morose that it is only when she is nearly killed by Thomas that her depression lifts, enabling her to face the thought of resuming her life. Tremas kept extensive gardens on Traken whilst Nyssa's mother was alive.
The TARDIS's translation net operates so swiftly that it is difficult for the Doctor's companions to differentiate between thought and speech. Because Nyssa was an aristocrat on Traken, it translates her speech into French, the language of the courts in thirteenth century England.
Links: Terminus. Nyssa recalls visiting England during the seventeenth and early twentieth centuries (The Visitation and Black Orchid, respectively). Nyssa recalls the events of Logopolis (see Goofs) and The Keeper of Traken. She remembers killing Cybermen (Earthshock).
Location: London, 1346 AD; Nyssa's Home on an unnamed planet, 3488 AD; Oxford, 1278 AD.
Future History: Nyssa left Terminus six years earlier, dating that story to c3480 (Terminus). By 3488AD, fashion viruses are in use in the system where Nyssa lives, which allow people to change their skin pigmentation. The planet she lives on has weather control and Home has extensive control over her immediate environment, including the ability to seed her bath with exfoliating nanomachines. The Tet-Gen Confederacy and the Jamlinray system have a cease-fire agreement, but conditions in the Staktys system are straining this treaty the system is racked by famine and the Jamlinray authorities are refusing refugee status to Staktys citizens. The system in which Nyssa lives has treaty obligations to Staktys. Nyssa says that by her adopted era Shakespeare's plays are still known of, but only in translation [this is only true of her sector of space - see Theatre of War].
Unrecorded Adventures: The Doctor returns Nyssa home between the end of this story and The Face of Evil [although it is possible that they make other trips first]. The Doctor suggested the line 'to exist or not to exist' to Shakespeare, which the playwright changed (to be or not to be) (see City of Death). The Doctor may have met Brunel, noting that he smokes the vilest cigars.
The Bottom Line: A thoughtfully written historical murder mystery, which benefits from excellent characterisation of the post-Terminus Nyssa and a restrained Fourth Doctor. The supporting characters are well portrayed and the period setting is well captured, although the novel is slightly marred by the unnecessary inclusion of the aliens. The essay at the back, in which Darvill-Evans describes the research he undertook in the writing of the novel, is fascinating.