The plan is refined and strengthened, contingencies are set to deal with various ways Thorn could've strengthened his defences, Mortal receives a very interesting email concerning one of their mother's contingency plans—namely that her assets have all been either frozen or transferred to Mortal themself, so Thorn doesn't have access to them -, and when Mortal and Promise judge there's nothing more to be gained from further planning they (eat dinner, sleep again, have breakfast, go over the plan once more when it's not completely fresh in their minds so they see if they come up with anything new, purchase Promise a mobile phone and a power generator to charge its battery in fairyland, eat lunch while Mortal teaches Promise how to use that, and) part ways.
On this side of the veil, Mortal gets to work. The first step: getting at least two safe houses, one for their HQ and the other for contact with the place near Thorn's court. They end up getting five, for redundancy's sake. The main HQ is near Seattle, the other four safe houses are in Greece, Russia, Japan, and Argentina. That, and getting the necessary existing equipment, is the easy part—you end up with contacts of the relevant sort when the bulk of your wealth comes from the kind of thing you can get with sorcery. The hard part is getting the various specific bits of technology that don't exist yet, including the the many types of trap and ammunition they'll need. Processing fairy voices with software turns out to be a dead end—apparently they're weird magical superpositions of sounds that make software go ?!?!?!?!?—but everything else, as agreed upon, can be made to spec nicely enough.
It'll take a couple of weeks beyond the one month for everything to be ready. Mortal hires someone who talks very fast.
And after the ball's going, there's not actually much for them to do with their time. They fret about details of the plan, order more redundant pieces of stuff (especially the to-spec stuff, not being mass-produced means they'd better have a lot of it to start with in case anything goes wrong), and have various antsy and anxious and calm and relaxed and terrified and panicked moods. A month is a long time...
At one point it occurs to Mortal that Promise might've decided to run away and not help, and then they'd never see her again and that would be terrible, and why would it be terrible anyway? It's just some fairy, fairies are evil, one must remember that. Even though she wasn't, of course, she was smart and resourceful and moral and ridiculously hot, and if they never see her again she'll never order them again and the tingly feelings won't ever happen again. Except what the heck, what are they even thinking? The answer, of course, is that they want to see Promise again. Why? To save their mother, of course. The only reason being ordered like that felt good was because Science. Of course. Of course.
The month passes—
"The problem is that the more vassals we get, the harder it is to keep the lid on, and we need to be very careful here." Button. "If you wanted to give Sand an invisible object about the size of a marble that he had to insert into his ear as well as instruct him to act in such a way as to minimise the possibility that anyone would find out about this object, what would you do?"
"And could you arrange to do that quickly and stealthily enough that no one else would be likely to notice?"
"Assuming I can order him once he has the object in his ear, do you believe you'd be able to pull this off without being seen by anyone else?"
"Suppose I could inform you of the locations of any surveillance fairies you don't know about?"
"How are you going to find the surveillance fairies? They're probably tucked away, not jut invisible," says Promise.
"Hmm, good point, I was thinking of an infrared camera noticing invisible fairies... Well, presumably any surveillance fairy would have to poke their head out to actually surveil? Unless you mean something else by tucked away?"
"Behind glass, maybe, would it see through that? Or an invisible piece of wall?"
"Glass has its own thermal signature, actually, it's not transparent to infrared, and presumably an invisible wall would be as visible in infrared as invisible fairies are. They would be transparent to radio waves but no one's managed to make radars sensitive enough for that yet, and fairies are invisible to ultraviolet. Why would they use an invisible wall, though?"
"But I mean, why use a transparent barrier like that at all? Expectation that we'd use something like an infrared camera? We can certainly detect invisible walls by just comparing the infrared image with the visible light image, and just avoid suspiciously positioned glass locations in general."
"...because the surveillance fairy is indoors intended to supervise the outdoors," says Promise, "they don't need a more complicated reason."
"Do we need to worry about a fairy supervising the outdoors, though? Presumably Verve wouldn't need to do anything noteworthy outdoors."
"We don't know how many surveillance fairies they might be or what they'd be monitoring."
"Right, yeah, I just mean, we won't necessarily need to worry about all possible surveillance fairies. And given that Verve doesn't actually know whether they're even there, they'll have to be unobtrusive enough to not be noticed."
"Yeah, but they do walk around, any invisible walls will need to be positioned in out-of-the-way places, and conversely invisible walls in out-of-the-way places are likely to contain surveillance fairies."
"I'd imagine tucking them under an eave or something - you know fairies can be very small, right?"
"Yeah, Verve did mention one the size of her hand. In infrared, anything that emits heat really stands out, though." He opens a feed from one of the infrared cameras looking into the room they're in. He presses the button: "How long do you usually stay around in any one court site?"