Roots: The Big Lebowski (Guerrier's model for Amy's learning behaviour), Buffy the Vampire Slayer, Rudyard Kipling (The White Man's Burden), the Doctor tells himself (badly) the story of the Three Little Pigs George Orwell's 1946 essay "A Nice Cup of Tea." The Golden Ratio.
Intertextuality: Zara and Zink's activities on Eratoon are detailed in the Seventh Doctor-era Companion Chronicle The Prisoner's Dilemma. The core idea of this second Key to Time quest, that of disastrous cosmic events being the result of the hasty and botched previous reconstruction of the Key has existed as a fan theory for some years and was published in the original Discontinuity Guide (see below) Pre-Warrior Mars with its floating cars and quasi-mediaeval culture would appear to owe something to Gary Russell's Radio Times Eighth Doctor comic strip Descendance. Magnetic drives in Martian vehicles first appeared in Lance Parkin's New Adventure The Dying Days.
Dialogue Triumphs: "That's the problem with power - it's always easy to break things, but mending them takes skill."
Continuity: The rain-swept planet Chaos [see; Links] is the farthest location in the future and exists only sixty-six minutes from the end of time.
Amy and Zara are living Tracers, blank, mortal vessels created by the Grace with inherent and complementary natures, but little free will and a blank slate personality. Each carries a satchel with its interior in a different dimension and powers this Universe can barely conceive - Amy's nose tingles in the presence of a segment to the Key. Three segments are recovered (or referred to) in this story: one in the form of a spaceship's compass, one held by Zara and recovered from Eratoom (see: Intertextuality), and the capstone of the great pyramid on Mars. The Key can rewrite matter, change the state of quanta and start and stop the universe.
Mars is first visited here as a pre-Warrior society; their culture includes beadles and magistrates including Lord Isskar. The equatorial city featured is nevertheless fortified and comprises shell-like buildings for reflecting the sun's heat, canals, pools and cooling fountains and nearby construction is underway on a series of pyramids (there is no suggestion of Osiran influence) with each stone a metre in height. There are ice caps to the north, and technology includes flying cars which are operated by a magnetic drive (see: Intertextuality) and are highly manoeuvrable. Martians are sensitive under their shells and show affection by kissing. Females are physically distinctive (or are apparently so in this era) The Doctor assumes that their Martian pursuers are swift of foot due to the low gravity of their natural environment. The Martians refer to themselves as Ice Warriors later in the story (i.e. much farther into their future - sixteen thousand years by the Doctor's reckoning). Four of them can carry the TARDIS.
Magistrate Isskar was present and witnessed Lord Izdaal step out and die, facing the Red Dawn (see: Links). He spends millennia in suspended animation travelling thousands of light years in pursuit of the Doctor, Amy and Zara. His ship has a strongbox employing the same empathic lock technology as that in Izdaal's tomb; similar to a warp drive technology, it places its contents outside normal time and space.
The district planet of Safeplace is inhabited by the Valdigians, a civilised insect-like species and descendants from refugees of past war. Traditionally their system of governance is a monarchy, but their laws decree that a King can only rule from the age of 23. As the people don't want to be ruled, they elect a child who agrees to stand down at the age of 22 in return for a generous pension. This way they never actually have a King in power and the provisional government put in place in the interim provides all the leadership they need.
The Laboon Constellation previously consisted of 47 planets, but was reduced to 46 after a war with the Cybermen. Its star system is oriented around a red dwarf.
Harmonius 14 Zinc's designation singles him out as belonging to the Commune of the White Sun. He has a time ring that he picked up on Barampa where it was believed to be purely ceremonial and was kept in a museum.
The Doctor is currently travelling with Peri, but has 'lost' her at the beginning of this story. In his pockets he carries a set of chattering false teeth, which is he surprised to see he still possesses, and a pack of snake anti-venom. He says he has visited Mars before (see: Untelevised Adventures), including with NASA (see: Links), but each time was after the Ice Warriors had left the planet. He says he has seen examples of the Golden ratio everywhere in the universe from sequences of DNA to the hearts of stars (the Grace use it too).
Links: The Key to Time stories, particularly The Armageddon Factor (the fake segment in the key which led to its corruption), Red Dawn (the Doctor refers to the events of this story, also references to Martian shell buildings, empathic technology and the death and tomb of Izdaal - also re-enacted in Deimos), The Ice Warriors and Pyramids of Mars. The One Doctor (mention is made of a 'Jelloid' prison) The Doctor says that he "knew a Zoe once" and remembers when Nyssa first tried chocolate (The Elite). The rain planet where the Doctor meets Zara is unnamed here, but given the name Chaos in The Chaos Pool.
Untelevised Stories: The Doctor participated in an archaeological dig on Mars in "twenty-five sixty... something". He also spent a week in the company of the Commune of the White Sun, enjoying their zygma tree-houses only to be imprisoned and threatened with torture for blocking out their sunlight.
The last time the Doctor was in the area of Safeplace the whole galaxy was at war, several generations before the signing of an armistice.
Q.v. 'Ice Warrior History', 'The Guardians and the Key to Time'
Location: The Planet Chaos (see: Links), Mars, Safehaven.
The Bottom Line: "It's a retro thing"
Big Finish adds to its world building referencing Red Dawn's early Martian history, while setting the foundations for its stand-alone Graceless series. There's a lot to juggle, and not all of it works (the Valdigians are stock BF insect aliens complete with annoying voices), but the twin foils of Zara and Iskaar are well conceived and written with style. Tracer Amy (they never thought to call her Tracey?) too, has promise as a temporary companion. In all, a strong start for something of a gamble of a miniseries.
THE GUARDIANS AND THE GRACE
The Key to Time was built by the Grace, powerful beings from outside Time, and beyond the Doctor knowledge; he speculates that they are 'big, peaceful and pan-dimensional' elemental beings. Under the power of the Grace are the opposing Guardians of Time, reinterpreted in this series as agents of chaos, self-will and freedom (Black) and order, control and 'bureaucracy' (White). In this model the question of good and evil isn't addressed so much as the absence or dominance of one half of the Guardian's shared influence - both are needed in balance, and yet both are locked in an 'eternal struggle' for dominance throughout eternity.
The Grace can momentarily stop time, an aspect of their own finite powers. Like the Guardians, their power weakens when the Key is corrupted or incomplete. As the Doctor's previous mission inadvertently destabilised the Key with a false segment (The Armageddon Factor) the Grace use their powers to create twin, balanced living Tracers (one for each Guardian) to collect and reunite the key's disparate segments. As new beings the Tracers are nameless and lack an established personality, accruing traits and values through exposure to those around them. In the Key2Time series much of the conflict arises not from the antagonism shared by the Guardians, but by the corruption of Zara (turning her into a selfish being) and Amy (less elfish, though no less of an individual, seeking her own independence.) Freed by the Doctor's intervention at the end of this series Amy and Zara escape the Grace and seek new lives while being pursued by their creators, as heard in the independent and otherwise unconnected Big Finish series Graceless.