Roots: There are references to The Chronicles of Narnia, Nigella Lawson, Angus Deayton, Pulp Fiction, Children's Favourites, James Bond, Threads, Wildlife on One, The Beatles, Mozart, Rogers and Hammerstein, and A Christmas Carroll. Erasure, for whom Jonathan Morris works, are also mentioned. I'm told that Bishop, who turns out to be an android, is also the name of the android in Aliens and Alien3.
Goofs: It is suggested that the Doctor would like to go back and prevent Sabbath from taking his heart, but in doing so Sabbath saved his life - the heart was killing him.
Continuity: The Clock-faced People live in the Time Vortex but are evacuating to escape Sabbath's allies. They take over human hosts by granting them the ability to alter their own past, correcting mistakes - this is an irresistible temptation, but automatically erases their timeline since it unravels their history. This allows the Clock-faced People to take their physical place. Humans perceive this as a transformation of the victims into clock-like beings, although apparently this is purely perceptual. They can reverse time for up to two minutes, and make changes in that period, thus allowing them undo bullet wounds. Their host bodies are otherwise still vulnerable and the death of their host bodies destroys them the Doctor wipes out the ones in Isolation Station Forty, by releasing mustard gas into the atmosphere. By the time the gas takes lethal effect, an hour has passed and they cannot reverse time far enough to avoid fatal exposure. They are dependent on the breach into the Time Vortex created by Paterson's experiments, which serves as their lifeline they all die when the Doctor seals it.
Sabbath adopts the guise of Mistletoe, which he describes as "An amusing grotesque. A theatrical device. A facet of my being, as viewed through a looking glass, you might say." The disguise appears to be similar to those used by the Master, in that it is clearly more than a mask and costume (see Castrovalva, Time-Flight, and The King's Demons). Since The Adventuress of Henrietta Street, he has acquired influential allies, who are clearly time active - they have been taking control of the Time Vortex, and were at war with the Clock-faced People. Sabbath agreed to create a scenario to facilitate their disposal, and decided to use the Doctor as his weapon of choice. He informs Anji and Fitz that he molded their perceptions as part of this scenario, thus explaining why the Plutocrats appear to use equipment from twentieth century Earth. He also insists that everything that happened on the colony is real, but is it not clear exactly how much of it he instigated - he probably programmed the actuaries four hundred years earlier, and may have even instigated the war in the first place. The Jonah appears as a door in a wall in Station One. Sabbath tells Anji and Fitz that his allies take a special interest in the Doctor, and that he is too important to Sabbath personally for him to let the Doctor die [if they are linked due to the Doctor's hearts, Sabbath might be affected in some way if the Doctor dies]. Following the destruction of the Clock-faced People, the Time Vortex is controlled completely by Sabbath and his allies.
It is hinted that close proximity to his other heart causes the Doctor some distress - he frequently grabs his chest in pain when he close to the disguised Sabbath. When the Clock-faced People attempt to possess him, his greatest regret turns out to be letting Sabbath take his heart (The Adventuress of Henrietta Street). He automatically tries to use his respiratory bypass system, forgetting that it is dormant (Hope). He carries a silver-backed mirror in his jacket pocket.
Fitz cannot remember what his childhood bedroom looked like (see Interference). He was sixteen before he first saw a banana and he used to watch Flash Gordon and the newsreels at the cinema on Saturday morning.
Anji starts reading a paperback book called The Worlds at War whilst in Isolation Station Forty. She has had anger management classes. She is still sensitive about her betrayal of the Doctor to Silver (Hope).
Travelling into the past requires more energy than travelling into the future, since it is uphill. A master control on the TARDIS console can cause a complete systems shutdown, which deactivates every circuit in the ship, including those required for life support. Proximity to the chronomium-based time travel experiments of the Plutocrats causes the TARDIS to fight so hard to get away that she is in danger of tearing herself apart [presumably it is the actual time rift that drags the ship off course and threatens to destroy it]. The TARDIS contains a room described by the Doctor as the chess room (where, naturally, he and Fitz play chess), which has the decor of an Oxbridge college room and features a crackling log fire. The roundels of the chess room are stacked with paperbacks. The Doctor keeps a torch in a locker in the base of the control console.
Chronomium is a time-active element that displaces time around it. In its unprocessed form, it merely slows down time around it, but it can be refined and used to accelerate time, achieve time travel, and protect against temporal fields (both accelerated and decelerated time fields). It is lime-green in colour.
Links: The TARDIS has been travelling for two days since leaving Endpoint (Hope). Anji tells Fitz that she isn't keen to see him naked again, and reflects that nothing could be weirder than the planet of the poodles (Mad Dogs and Englishmen). Sabbath's entrance, as Mistletoe, walking across a battlefield in a bowler hat and carrying a clipboard, was first seen in the time engine scene in Father Time. The Doctor mutters that not even the sonic screwdriver will get us out of this one as an aside, in a nod to The Invasion of Time. The Doctor's erratic memory throws up the information that not being able to change the past is the second, no, first, rule, which is a running gag between the Fourth Doctor and Romana in Morris's Festival of Death.
The Vortex Wraiths in The Slow Empire were presumably evacuating the Time Vortex in order to escape Sabbath's allies.
Location: Isolation Station Forty and Station One, an unnamed planet, date unknown [the future].
Future History: The Plutocratic Empire franchised the development of the planet as a colony, holding a lease on it in return; when the colonists defaulted on their payments, the Empire decided to repossess the colony and sent receivership forces to invade. The resulting war between the Plutocrats and the Defaulters lasted for four hundred years. The Plutocratic forces on the planet are controlled from Station One (the original colony town) by the actuaries, seven automata designed to produce the most economically acceptable outcome. In fact, through carefully placed android agents, the actuaries control both the Plutocrats and the Defaulters, and have been maintaining the war as a stable system in order to generate revenue and dispose of financially non-viable citizens of the empire. After four hundred years of war, the Plutocratic Empire itself collapsed, leaving the actuaries without new recruits and uncertain of their function they are unaware of the actual purpose of the war, and are about to cease trading for good (i.e. end the war) because no new soldiers are arriving and because they have nearly worn out and are on the verge of ceasing to function. Because of this, they ordered the Plutocrats' time travel experiments, intending to travel back and find out what their original instructions were before, as well as changing the outcome of the war and trying to prevent the Empire from collapsing. It is not clear how much of this is Sabbath's doing, or even if the Plutocratic Empire as described really exists.
Unrecorded Adventures: The Doctor purchased a leather bound notebook in a London bookshop in 1938.
Q.v. Time travel.
The Bottom Line: A tense and unremittingly grim horror story, which makes surprisingly effective use of the base-under-siege cliché. The fact that the Doctor has to resort to using mustard gas emphasises the desperation of the situation, and the moment when he turns round to reveal the face of a clock is spine tingling. The twist is well concealed right until the end and promises big things on the horizon.