Roots: Stephen Fry's Making History and other Nazi-based speculative fiction. Space: 1999 (a moon base with 'Eagle' craft), Minority Report, The Wizard of Oz ("click our heels together and chant 'there's no place like home'")
Intertextuality: The Selachians were created by Steve Lyons and were initially mentioned in his Virgin Missing Adventure Killing Ground, first properly appeared in Lyons' Second Doctor story The Murder Game, and returned in The Final Sanction (also featuring the same TARDIS line-up), both for BBC Books.
Dialogue Triumphs: in episode two's cliffhanger.
Double Entendres: "That Selachian doesn't look like a guard... more like an escort."
Continuity: Selachians are a race of heavily-armoured shark creatures equipped with plasma weaponry and bio-organic transmat technology. They are water breathers, and therefore their armour provides the correct living environment for the wearer (and is fortified against puncture from heavy weaponry excluding their own). Their space ships are black with dorsal fins and shark mouth decals across the bow. The Selachian race is for the most part motivated by past offences against them perceived and real by other species including humans; they are by nature distrustful, having planted an agent on the Moon to spy on the Doctor. Selachian transmit devices feed off the blood of their wearer.
Having secured the Doctor's TARDIS Klein travelled back to 1944 and after several attempts 'corrected' her timeline. The Axis won World War Two and a hundred years on still control the Earth, having exploited time technology to further their own technologies from as far as the 31st century. Technology described includes an enviro-dome on the Moon, "Eagle" space craft, photonic energy weapons and construction that incorporates dalekanium in the lunar dome's bulkheads, the 'Dalek Wars' being part of this timeline's modern history and within the lifetime of some. Klein herself has risen to the position of Oberst and a temporal agency set up under her command. Curiously she never returned for Jonas Faber and they were never reunited.
Having lost his TARDIS the Doctor in Klein's timeline has made contact with the Selachians and, promising them temporal technology advanced their technology from 1944 (Earth equivalent timing) to send six fighter ships to the Moon a hundred years later. Enlisting the help of an Earth woman called Rachel Cooper he has apparently had several adventures with the erstwhile garage worker, encountering Daleks, Sontarans and Autons and possibly also Zygons before embedding her in Klein's timeline to aid in the Selachian invasion. Six weeks previously he was captured and imprisoned on the lunar base, and has been visited by Klein at least four times. His new ziggurat-shaped TARDIS has a perception filter on it (see: Links) and is clearly a different model from 'his' vehicle, having black interior walls and an extermination chamber which will remove Klein entirely from the timeline [it may be inferred that this is a customised vehicle given to the Doctor by the Time Lords who have 'judged' Klein in absentia]. The 'true' Seventh Doctor's consciousness merges with that of his counterpart whilst in captivity - presumably as a result of either Klein's continued adjustment of history, or some unexplained outside influence [the Time Lords?].
Presumably the TARDIS in the story's epilogue is the Doctor's own, restored to his ownership with the erasing of Klein. In the restored timeline itself Elizabeth Klein is a UNIT scientist who knows of the Doctor, but has not yet met him; to her the Doctor's presence at Colditz Castle is a matter of speculation and nothing more.
Rachel was 21.
The 'restored' Klein was born in the UK to German parents, raised in the time of war, gifted with an enquiring mind, a prodigious intellect and a need for order. She knows of the Doctor as having been in Shoreditch in 1963, The Shangri-la holiday camp 9159, Kenya in 1953 and (unconfirmed by UNIT) Colditz Castle in 1944.
Location: The Reich's moon base, Klein's timeline 2044
Links: This story immediately follows Survival of the Fittest, although at least three days in the Doctor's timeline have passed, and an indeterminate amount of time has passed for Klein, Rachel and the Doctor's alternative timeline counterparts. The Doctor recalls joining Klein against the Cheylis in Kenya (A Thousand Tiny Wings) and the Vril (Survival of the Fittest), and reminds her of the fate of her lost love Faber (Klein's Story). Dalekanium. Rachel mentions Daleks, Zygons, Sontarans and Autons. The TARDIS' perception filter is introduced in Utopia. Klein's challenge of "end my life" to the Doctor is a reference to his similar words in The Happiness Patrol as well as Mordred's to the Doctor in Battlefield. Among the 'restored' Klein's references to the Doctor's Earth activities is that of the Shangri-La Holiday Camp in Wales (Delta and the Bannermen). Boomtown (perception filters).
The Bottom Line: "Everyone has to die sometime. I accept that, Doctor. What I can't bear is the thought that I might never have lived."
Despite the distraction of the Selachians, and the shaky performance in Lenora Critchlow's Rachel, this is a big story with grand ideas and a clever script. In all, a stunning end to Big Finish's most consistent trilogy to date. As Klein's irredeemable dogma endures, Lyons gives us no easy side to vie for, with a classic, manipulative Seventh Doctor who issues a cold sentence - and sacrifices in his triumph a companion he never knew. The coda's rather nice, too: you could almost see these two working together.