Roots: The Guldreasi's description of themselves mimics "word made flesh" (John 1:14), while their form of negotiation ("agree/disagree?") mimics opinion poll phrasing.
Double Entendres: "Everybody else's mothers are embarrassing at home. Why does mine have to do it in public?"
Dialogue Triumphs: 'Eight? How did you manage that? That's just throwing them away!'
"You're not hope. Hope is jam tomorrow, the starlight in the storm, the candle 'round the corner. You just feed on the fears of others."
Continuity: Two thirds of the planet's population were killed during the Dalek invasion, and technology was reduced to the levels of two centuries previous. Earth is overseen by an Earth Council, headed by a President housed in Kew, with whom Susan has some influence. Despite the slow progress suspicion of extraterrestrials continues with the recent invasion being for many of the population the only experience of extraterrestrial cultures in living memory. The 'watch', a group aligned with Earth United actively resists anything alien in origin - it is implied that their burning of remaining Dalek technology may in part be a contributing factor to large urban conflagrations.
During the Invasion the Lunar colonies were cut off from Earth; Earth can broadcast to them, but not vice versa. Technology has been thrown back "two hundred years" (according to the story blurb), and examples in evidence includes cars, helicopters and typewriters. Slythers are rumoured to still be at large (Alex says nobody believes those stories any more). "Robo-head" and "alien" are terms of derision, while xenophobic organisations like Earth United have emerged as pressure groups to prevent other alien species from threatening Earth.
The Guldreasi are a conceptual species 'thought made flesh' as Hope puts it. They trade in slaves.
Susan (now Susan Campbell) works for the Earth Council. She hasn't seen the Doctor since his Fifth incarnation and is appalled that he's already used up so many lives. She identifies as Gallifrean and has two hearts, but has seemingly aged in 'real time' [a consequence of her separation from the TARDIS?] The Doctor expresses surprise that she could have borne a child to a human (does this put paid to the idea of his being half-human?) Her late husband David (the cause of his death is not given) is still regarded as a hero of the resistance and rebuilding afterward. She has built a transmitter and is covertly making contact with extraterrestrials in her search for aid in Earth's recovery.
Susan's son - and the Doctor's great-grandson, Alex (David Alexander Campbell, born ten years after the Dalek occupation) is seventeen and a student at Bristol University. He only has one heart (on his father's side.) It is evident that he knew his father while growing up, but is not aware of his own genetic heritage.
The Doctor has picked up Susan's transmissions through the TARDIS. Whilst in the custody of the Guldreasi the TARDIS seems to attempt to make its escape [perhaps the HADS is operating?] His offer to have Alex invested in the Academy on Gallifrey suggests that his influence is still significant there (or at least he thinks it is, even after the events of Zagreus. Did his part in Morbius' defeat repay all debts?)
Untelevised Adventures: The Doctor was present at the opening of Brunel's Clifton Suspension Bridge in Bristol, 1864. He implies that he and Susan met Genghis Khan (see: Links)
Links: This story's title is, of course a nod to that of the series' first episode An Unearthly Child, concerning Susan. The Dalek Invasion of Earth. The line "on his father's side" is Marc Platt's rather cute reference to the TV Movie (as well as the issue of Susan having a half-human child.) Susan refers to having met the Doctor's Fifth self (The Five Doctors.) The Doctor's mention of having met Genghis Khan is also noted by the Ninth Doctor (Rose) and Sixth Doctor (City of Spires) The Iborians, an alien race who come to Earth's aid at the end of this story are mentioned again in Lucie Miller.
Location: London and Bristol, 22nd Century (thirty years after invasion began)
The Bottom Line: "There are aliens everywhere Alex, some of them never left"
Routine, sadly. Although Big Finish routine is better than most. There are beats here - Susan's isolation, Alex's curiosity about his beginning, that could have played stronger, and the McGanns' chemistry too is strangely cold and underdeveloped. But better things will come from this Time Lord pairing.