Besides, a look at ancillary characters without the Doctor about and see how they cope in a situation that greatly needs him sounds right up my street.
The UNIT storyline, feels exactly like a Pertwee era story about the investigation of a crashed ship, except the Doctor's missing. In New Who this could easily be a Doctor-lite episode where we concentrate on what people in the Doctor's orbit do when the Time Lord isn't about. This story proves what a shame it was that it was never experimented with earlier. Seeing UNIT deal with a mission without him would have been incredibly intriguing.
Having the Brigadier bring in Ian and Barbara to fulfil the Doctor's normal role is great, after they've acknowledged that they can't get hold of Liz Shaw. They also strike a nice balance. They've been back on Earth for ten years. They're used to normal life again. Diving back into situations that the Doctor specialises in is a little exciting and a little daunting all at the same time. It's really good to see the life of a companion so far removed from their actual time on the TARDIS.
On the other hand you have the story of Detective Boucher and the bank robbery, which feels like a gritty 70s cop serial. It's a stark contrast to what's going on back with UNIT, as we have shoot-outs, grasses, drinking coppers and a scene straight out of The Godfather Part 3. Even the first meeting of the Master fits it perfectly, with him practically running the prison he's been thrown into.
It could all fall apart, except McIntee does a fantastic job of switching between the two. Yes, they feel completely different, but they're also the same story, as you read on aspects from each slowly start to trickle through. Once the crime story has been fully integrated, the remaining non-dimensional elements are dealt with harshly to remind you just how this all came to be.
The Master is one of the points that hold the whole story together, and as such we get a deeper look into the character than we have before. It also makes his various appearances throughout this era of Who all the more feasible, by just giving us this insight into what the Master is doing when he's not mid-scheme against the Doctor, because he had all these things behind him, just to keep things rolling and in money. The idea he views it all as an inconvenient necessity too just sells it even more.
The Master is one of the bad guys I can't make my mind up about. However, I love the concept of an evil Doctor and here that mirror is held up in even starker contrast. By going to UNIT and getting the Brigadier to reluctantly have the Master take the science role he literally becomes a violent unscrupulous Doctor. All those people that talk about how they'd prefer a Doctor who's a bit more violent get to see what the UNIT-era would have been like if that was the case. It's a fun distraction, but Who would never have lasted with it.
Of course there's the Inferno tie, which as it became more obvious I was uncertain about. Apart from not liking that particular serial, it's also quite self contained. Until McIntee brought the science out and made it make so much sense. Combined with him giving a very brief history lesson in the world, it adds a nice couple of layers that were missing from the original story. It also uses various aspects from the show's run on Earth to form a single narrative for this other world.
Throw in a young Harry Sullivan getting his start in UNIT and this feels like an important piece of the puzzle that makes Doctor Who more a tangible universe than a show just trying to meet deadlines.
While the The Wheel of Ice felt like an additional entry to the franchise just because the author could, The Face of the Enemy fills in a lot of gaps, yet managing to stay entertaining. By acknowledging the personal lives of everyone, something the show ignored, it breathes life into them, and the whole era. Not only have Ian and Barbara got a child (who we don't meet as they are responsible parents left him with his grandparents) but they lost their jobs for disappearing for two years, but thanks to connections with other people the Doctor met both eventually landed on their feet. The Brigadier is worrying about his own personal life in the aftermath of Barbara's supposed death. It bring a lot of humanity to the players that was missing from before
Using the continuity to frame a fantastic story, instead of being a slave to it and just being a set of tick boxes of Master, Inferno, Ian and Barbara, anything else that would help. This book most certainly is an essential addition to Who.