We're really on a roll with these past few books. This one is stellar, if a bit convoluted and too dependent on exposition. We have the return of the Celestis and more information on the War against the Enemy and what it has done to Time Lord society. Delicious, delicious lore, and some nice character moments for the team as well.

Putting things in order, One's and the Hermit's goal is to destroy Mictlan, because Mictlan might make the Swimmers (enormous predator universes) become aware of our universe and come and Get Us. Their complicated plan goes more or less like this:

• They create the version of history where Lovecraft's Elder Things actually exist and live on prehistoric Earth.
• War-time Time Lords come to prehistoric Earth disguised as Elder Things, unaware that those things should not exist, as part of a mission to break the timeloop around Planet 5. That's where the Fendahl is trapped, and the Time Lords want to free it to use it as weapon in the War.
• One wants them to succeed in breaking the loop, because he knows that what's actually in there is the Predator of the Fendahl, a things that eats memory and meaning, and that can eat Mictlan.
• After the Predator eats Mictlan, One plans to use the War TARDISes to cut it off from the Universe, and send it away, thus making sure the Swimmers never come and *nhac* eat our Universe like a cookie.

The Doctor is not part of the original plan, but One makes it work anyways, using him to command the War TARDISes. One's going with the (correct) assumption that the Doctor would follow his plan (which he telepathically implants in his brain), since it cutting off Mictlan is the only thing that can save the Universe from the Predator.

It's all very, very cool. The writing style is compelling and entertaining, and the character's voices are captured well. My main complain is that One's plan is very convoluted, and could have gone wrong at many, many points. If the Doctor hadn't managed to diverge most of the TARDISes, they would have all been destroyed when they hit Planet 5, and then what? How would they have gotten rid of the Predator? Etc. etc. In the third act, there's quite a lot of exposition from One to Xenaria as he monologues about his plan. I can't help but feel like it could have been told in a more dynamic fashion. It's not so bad, as the other characters are having some very action heavy moments at the same time, but still.

Fitz is great as always, and gets to be the big hero and save the Doctor's life at the end. Compassion makes zero effort to be likable. She has a weird thing going on with the TARDIS, probably a consequence of the filter the Doctor put in her receiver. She talks not only to our TARDIS, but to the War TARDISes as well. Marie (2nd best TARDIS ever) saying 'you didn't realise who she was?' to Homunculette sounds ominous as hell. I'm guessing Compassion is not gonna be one of those companions that meets a nice guy and leaves to get married, uh? All that makes her an interesting character, but she doesn't feel like a companion at all. It's pretty funny how she really doesn't care about anyone, including the Doctor and Fitz. The girl's lack of empathy and morals is driving the Doctor crazy. His pet project now is making her a Better Person, a better human, according to his own idea of what that means. Can't wait for that to blow up in his face!

I can't believe we got Homunculette (first seen in 'Alien Bodies') back!!! Loved seeing him again, loved Marie's cameo at the end, and looooved every bit of war time Gallifrey we got. It's brutal, the Doctor is horrified, and so am I. They're force-regenerating to take up heavily modified bodies as disguises; Xenaria believes the Doctor to be a general because he's in a normal body, meaning that most soldiers have been modified. They're sending fresh out of the looms Time Lords straight into battle. Renegades have been drafted in the war effort (the Rani!). The War TARDISes are aggressive and angry, treated as slaves!  The landscape of the war is slowly taking shape: the Time Lords have made nine copies of Gallifrey, one of which has already been destroyed. The nature of the Enemy is still unknown. Faction Paradox is involved, but it's far from being the main antagonist of the Time Lords, and as of now, Mictlan and the Celestis are no longer players in war. The Doctor wants nothing to do with it, but keeps being put in situations where he's made to get involved. He stops the auction for the 'super weapon' in 'Alien Bodies', stops the giant Seal of Rassilon bomb in 'Interference', and basically destroys the Celestis in this one. And I'm sure there's more to come. I keep wondering what Romana's doing in the meantime.

The Doctor was really great in this story. A lot of good lines, a lot of great moments. His mantra of 'I'm the Doctor, I have walked in eternity. I have died many times. I have fought countless monsters. I have saved countless lives' to keep himself focused as the Celestis fiction machine thingy changed history was very nice. A lot of very good moments for him and the TARDIS as well, as when she vouches for him when the War TARDISes ask her if he can be trusted. It made me think of how angry she's with him in 'Zagreus'; thank God she's forgiven him for what happened in 'Unnatural History', or he'd be dead dead dead.

The Doctor spends 70% of this book covered in his own blood and being hurt by others (One and the TARDISes) or himself (to get the War TARDIS to activate their security protocols). It's all going on the list:

Memory Loss:1 (in 'The Eight Doctors')
Serious Injuries/Near Death Experience:11 (gets vampired 'Vampire Science', nearly drowns in the Thames in 'The Bodysnatchers', bomb+fingers broken in 'Kursaal', electrocuted in 'Longest Day', gets shot + severe blood loss in 'Legacy of the Daleks', nearly squashed by giant hydra in 'The Scarlet Empress', leg broken + slapped around by giant tentacled monster in 'The Face-Eater', stabbed 3 times in 'Unnatural History', electrocuted in 'Autumn Mist', broken arm + more in 'Interference' 1&2, broken wrist + near death in vacuum of space + more here)
Torture:5 (in 'Genocide', 'Seeing I', 'Unnatural History', 'Interference' 1&2, some more torture here)

I've actually read this book once before, many years ago; it was my first ever DW book. As you can imagine, at that point I understood little, but loved the vibes, and loved Fitz. Coming back to it now, I can say I still love the vibes and Fitz, and boy does it feel good to actually know (most of) what is going on.

Liked the ideas in this one more than the execution. Shout out to what I can only describe as an EightTARDIS sex scene

Note: I also posted this one onto GoodReads

‘Odd that you mention new words.' The Doctor's face was red now, capillaries swelling under the biomass probes of the stone. 'Gallifreyan's always been a static language, hasn't it? We go out into the universe and talk to everyone, but we don't take loan words back into our own tongue. TARDISes translate everything for us, or time rings, or things like your locket there. We don't have to integrate our experiences into words: they do it for us.' A vein in his neck began to pulsate, and next to it a warm glow spread through the blackness of the stone as if it were pleased.

A quick word of warning about this one. There's not anything too bad in it, but it is a very complex book. It ends with a "cosmobiological" paper. Perhaps you should read Alien Bodies and Interference first, although I didn't read those and understood most of what happened.
This is probably the best portrayal of the War in Heaven within Who. We feel the frenzy of the War-Time Lords, the pure frantic desperation which leads them to literally make deals with the Devil (for what is the Fendahl if not Doctor Who's version of Satan). You can also feel the raw fear and paranoia of the massive and unknown that the main villain of the piece.
I'd say that this is a good introduction to the arcs of the EDAs that come before it, but I was at least aware of most of them. At the very least, it explains Compassion and The Blue Angel pretty well.
If you enjoyed any of SB-J's other novels and/or enjoyed Hope, you'll probably enjoy this one.