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—
"Finding another knifewing in particular doesn't seem particularly high-priority."
"Well, no, certainly not, it probably wouldn't be one of the top twenty items of my priority list, but there are so many ways to use fairy magic to improve everyone's lives."
"Maybe I'll be in more of a science mood when Thorn's entire court is good and locked down."
"Locked down? And actually, yeah, what are the plans once his court has been secured and neutralised?"
"I'm going to have to interview them all. See how deep their loyalties go."
"Yeah, that's along the lines of what I was thinking. Vassalhood may be terrible but it sure does make interrogation much easier, and is much better than some alternatives employed by mortals."
"It's not like fairies don't do that. That's how Thorn got my name."
"I suppose if you don't already have someone's name," she sighs. "All in all I prefer obtaining my answers in a non-coercive manner, but vassalhood is an acceptable alternative under the circumstances."
And back to waiting, since Thorn should be arriving any minute now, if his prediction was accurate.
Thorn implements his plan.
Mortal tenses up like a violin string, and doesn't hover her hand over the button because, well, Promise's doing that. She merely watches.
Eventually he has the site sewn up, Blossom and all.
Promise sends him to get some sleep with an appropriately decorated order and relaxes when he goes as he's told. "Next site tomorrow."
Mortal stops fidgeting at about the same time. "I am very much not meant for this kind of waiting."
"It's fine, we're doing a good thing and hopefully only the first of a long string of good things we'll be doing." Pause. Catch up with what she's said. "I mean, if you want to," she amends.
"We seem to have broadly compatible goals and complementary skills and work together okay when you're behaving reasonably."
"The exact conditions that led to it are unlikely to come up again, and even if they did I would act differently."