Roots: The first chapter contains a blatant parody of Blue Peter. There are references to Newton, Nescafé, Enron, J. B. Priestly, Stephen Hawking, Feynman, Christopher Priest's The Prestige, Carter Beats the Devil, Whatman, Niels Bohr, Shay McNeal's The Plots to Rescue the Tsar, The Independent, The Daily Telegraph, and Baden-Powell.
Goofs: Nathaniel Chiltern from Camera Obscura is referred to as Nathaniel Ashe.
Dialogue Triumphs: The Doctor's off-the-cuff parting shot to the SAS soldiers: 'If you do turn out to be nasty, unpleasant people, you'll have me to answer to.'
Continuity: Sabbath adopts the alias Holiday, a corruption of Holy Day, and thus a play on his name. The Doctor notes that the alias Mistletoe is a similar play on words, since mistletoe is common at Christmas (Anachrophobia). The Doctor realises that Sabbath wanted the destroyed time machine from Camera Obscura for himself, to allow Curtis to walk back to Event One. Sabbath once more alludes to his mysterious allies, who have told him that time travel is dangerous because it splits off alternate timelines. He reluctantly admits that they might not have told him the whole truth, after the Doctor convinces him otherwise. Due to Sabbath's actions here, Curtis's death in 1894 splits off an alternate timeline, leading into the events of The Infinity Race.
The slow light trapped in the ice is from an o-region, an area of space so far out and isolated that its light hasn't reached the rest of the universe yet. This light houses an intelligent life form, which following the destruction of the ice in the cave, burrows down through the arctic tundra to the magma seeking a new medium in which to exist and merges with it, becoming the fire creature from The Burning.
The Doctor found the journal written by Fitz during the Hanson Galloway expedition of 1894 in a bookshop on the Euston Road in London whilst trapped on Earth. His second heart has completely re-grown itself (Camera Obscura).
Fitz is now thirty-three years old (see Camera Obscura).
Anji returns home for eighteen months, before being coerced into visiting Siberia by Siberia and thus meeting the Doctor again. She initially regrets leaving the TARDIS, since she is worried about whether Fitz survives his expedition, but by the end of this story she is keen to return home once more having resolved her issues. Anji's colleagues include Mitch, her best colleague who started work the same day that she did. Other colleagues include Josh and Graeme. She kept cuttings from future editions of The Financial Times and did extensive research whilst travelling with the Doctor into future finances; despite this, she doesn't use the information, preferring to trust her instincts.
The Doctor says that the TARDIS is made of fire in a sense (see The Burning).
Control appears, and authorises Hartford's mission to Siberia to investigate the evidence of time travel experiments (The Devil Goblins from Neptune, The King of Terror, and Escape Velocity).
Trix adopts the guise of the Grand Duchess, the supposed daughter of Alexei Nikolaivich. She knows Sabbath and is possibly a time traveller, although she doesn't seem to have her own means of transport. She claims to be old friends with the King of Tibet, but she might just be being flippant.
Links: At the end of the novel, Sabbath hints that he is going sailing, leading into The Infinity Race. Somebody leaves a model boat on the TARDIS console so that the Doctor will follow Sabbath. The note telling the Doctor to meet Fitz in St. Louis is mentioned (The Burning). Dave is mentioned (Escape Velocity, Hope). The Doctor recalls spending many hours in the reading room of the British Library during his time on Earth (Endgame). He mentions Spain (History 101). He says that he has faced people who became clocks (Anachrophobia), and bargained with fire demons (The Burning). Fitz remembers the acid seas of Hope. Fitz alludes to the grandfather paradox and quickly changes the subject having realized what he has just said (The Ancestor Cell).
Location: London, October 12th 1938 in two alternate timelines; London and Siberia, 1894; London and Siberia, 2001.
Unrecorded Adventures: The Doctor implies that he knew Alexei Nikolaivich.
The Bottom Line: A gripping thriller from Richards with a healthy dose of science fiction. As well as further defining the Doctor/Sabbath rivalry, it also serves to tie-up one or two lose ends from The Burning onwards. Hartford makes a suitably loathsome villain.