Roots: Ælfwynn, might as well be Xena, Warrior Princess, [Alfred]...who burnt the cakes.' Can't Buy Me Love. 'Professor Fester' is likely named after Magrs' cat (RIP). Reference is made to "A sorcerer and his apprentice". Tegan's historical obscurity posing as a nun recalls the fate of Robin Hood's Maid Marian.
Goofs: Aelfrith's mention of a 'wizard' and 'caravan' seem out of place, or not anachronistic (the first is an example from Middle English and even in early Old French the latter would be on the cusp of common use), not to mention threatening to draw and quarter Tegan and the Prof? As for the berserk scene...
Why does the sword go to Derby?
Why does Tegan hear the sounds of battle on seeing the sword's pommel jewel?
The Doctor says that travel in the vortex is not deadly (according to No Place Like Home humans exposed to the vortex wither and die in seconds.)
Technobabble: the Doctor's tracer fails when the signal fades - the time distortion breaks up. A "shadow-wave formation of a living pulse within a corporeal event indicates a living body travelling in time.
Dialogue Triumphs: "If I've learned one thing travelling over the past few years, it's that history has gaps in it..."
Tegan on the Doctor's time tracer: "Oh, and out he comes with a gizmo!"
"Turlough, put the kettle on..."
Dialogue Disasters: "Down, down, down, down, down with the laws of physics!"
"Buck up, Beaky!"
Double Entendres: "We saw him do it - it was incredible, anybody could have seen him!"[?]
"Aelfwyn - daughter, step away from that device!"
"You look whacked out, if you don't mind me saying"
Continuity: The University of Frodsham is apparently one of the most august institutions in the North. Its campus is on a hill, and it features a Mother Theresa Suite and a duck pond.
Philippa Stone's time machine scans the fabric of an object's history in four dimensions, attempting to latch onto its time track [this seems to be measured by the object's last recorded location.] Tegan and Bleak travel through time unguarded (compare their lack of harm with that of Mel in Catch 1782, also see: Goofs). They end up in Derby - so their travel is to an extent also geographical. Some time has passed for the Doctor and his companions since the previous story (Tegan says they've been cooped up in the TARDIS "for weeks.") Two days pass for Turlough before he is reunited with his travelling companions (wonder what he and Philippa did to pass the time..?)
Tegan (posing as a 'Professor Stimpson') proclaims the University of Wollongong "a dump". She doesn't like sherry. Tegan went to college briefly whilst studying to be an air hostess. She learned to ride a horse on her father's farm. The Doctor suggests that Tegan has picked up some form of time-sensitivity during her travels in the TARDIS
The Doctor says he built the TARDIS food machine.
Location: University of Frodsham, 1983, Mercia (including Yorkminster) 918
Links: The Butcher of Brisbane (Turlough mentions Findecker). Nyssa remembers to mention Traken.
The Bottom Line: "This is not historical!"
Janet Fielding gets the best role in the script, including a difficult accidental killing. Mark Strickson, however, doesn't do anything other than offer the occasional cutting remark, although his rivalry with 'Barry' is rather cute. Nyssa seems to disappear in this story, but this all seems to be resolved next time...
TRACKING THROUGH TIME
The Time Vortex, the Doctor explains in The Butcher of Brisbane, is an ever-shifting environment, therefore tracking temporal hazards such as chronon leakage [The Four Doctors] or the movements of a vehicle inside the Vortex - or indeed outside it from within the Vortex, is an exact science. Several times the Doctor has had to fabricate his own means of detecting the TARDIS of other Time Lords (e.g. his 'TARDIS sniffer-outer in The Time Monster) or his own. In The Lady of Mercia he carries a "time finder" to track Philippa Stone's time machine, while in Catch 1782 and The Book of Kells similar portable sensors can detect time eddies or time distortion associated with an object. Occasionally he must turn to more drastic measures: locating the TARDIS Klein has stolen is no easy task for the Doctor in Architects of History, while in The Boy That Time Forgot he employs block-transfer computation during a séance to trace the TARDIS Brewster has stolen from him, which he presumes to still be in the Vortex.
So, what sort of phenomena do these detection devices capture? In Prisoners of Fate we learn that a dematerialising TARDIS leaves a residual temporal trace (referred to as chronometric particles in Singularity) lasting up to thirty minutes, and in the same story the Doctor talks of his old Type 50 creating a 'time trail' in its travels. In Creatures of Beauty we learn that TARDISes emit warp distortion fields in flight - indeed, that time distortion is a visual phenomenon, recognisable as 'shards' travelling with the source of disturbance. Certainly, the 'spoor' [Dark Eyes] of the Doctor's TARDIS is distinct and strong enough that Philippa tone's machine can follow it even before it has been created by the TARDIS itself. Temporal vehicles which are not TARDISes also seem to create recognisable disturbance in flight: devices using zygma energy cause a drop in quantum pressure, creating Vortex turbulence [The Butcher of Brisbane], whilst Dalek ships have a space-time core which creates a disruptive chronon wake [The Elite]
Tracking the presence and movements of temporal phenomena is understandably a vital function of Time Lord technology. The Doctor's TARDIS seems to have its own mechanism, called a 'time tracer' in The Lady of Mercia and (if it is the same thing) a 'time finder' in Hexagora; and the TARDIS can detect other TARDISes and time vehicles in its vicinity as a matter of course [The Crimes of Thomas Brewster]. There is evidence to suggest that the Time Lords use these devices to track their own people through their TARDISes - as Straxis does in Dark Eyes (the aforementioned 'spoor'), and the TARDIS's 'LTD' button does in The Beast of Orlok - the latter specifically homing in on the Doctor's own heartbeats.