Cass.

Hello again, Cass.  The original four Time War boxes ended on something of a cliffhanger with the return of a version of Alex so there had be a lot more incidents between that and the cracked Doctor who Benny and River meet during Lies in Ruins (from The Legacy of Time).  Since it is a Time War, and history is in flux and with Rakhee Thakrar not returning as Bliss because her career's entering the next stage, why not a pre-meeting with the Eighth Doctor's final almost companion with the added mystery of how they can meet here and again in his future.  It's a variation on the theme of Charley Pollard, although without the companion knowing that the Time Lord will become such a big part of her future and trying to figure out her own solution.

Despite a couple of references in the dialogue we don't really receive an answer and its clear by the end that there are more stories to tell, probably with Cass 2, 3 & 4 in the production pipeline (which is why Big Finish have gone with a new title rather than simple called it The Time War Volume 5) (apart from when they do in the credits at the end of each episode) (I'll update the title of this post if necessary).  Will it be as simple as retconning The Time of the Doctor, so that he knows full well who Emma Campbell-Jones's Cass is even if she doesn't recognise him and he's pretending (which puts a new spin one her rejecting him outright).  Rule One: The Doctor Lies.  When he has to.

Meanwhile, Elsewhere

With very little to work from other than Time of the Doctor and the novelisation of Day of the Doctor (with which this is broadly consistent) Cass is the type of female friend the Eighth Doctor tends to swan through the universe with; independent, capable and well prepared to call him out when he's clearly bullshitting.  But it doesn't feel perfunctory when she's invited onto the TARDIS at the end - she fits right into the time paradox shenanigans.  The theme of the boxset seems to be stories which have resolved themselves before they've begun and I'll admit this took me two listens to really follow what happens and how.  The loquacious Hieronyma Friend feels like she was originally written to be River Song.  Perhaps Alex (Kingston not Campbell) was unavailable.

Vespertine

A never meet your heroes tale with a twist.  Once again, this took me a couple of goes to completely follow the action either because my inevitable cellular decay means I'm slower on the uptake than I used to be or jointless time is difficult to convey on audio where its difficult to have diagrams (or both).  It was a pleasure to go round again though.  The three leads have an easy chemistry especially Emma, and Sonny McGann who in the ten years since his last appearance has become a fine actor and the writers of these stories have really leaned into the enthusiasm for adventure this version of the Doctor's great-grandson has, every now and then even sounding like the younger version of his father who debut in these audios twenty years ago ("TARDIS manual, TARDIS manual ...").

Previously, Next Time

The inevitable Dalek story, although they're used in a much subtler way than in the previous Time War stories and although their plan has the ring of one of those earlier jargon soups, this looks at the effects of the scheme on a more human level, the Doctor faced with an impossible choice morally but not logically, of whether to wipe out an entire civilisation, no doubt foreshadowing the adventures of his successor.  I am slightly confused as to whether the effects of this retcon bomb's are only felt on this planet, the whole Whoniverse (explaining the incongruities in the Doctor's memory in the previous stories) or if there's some larger mystery which is yet to be explored.