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—
"Yeah, true. So, probably one of the singles. Any last requests, cold feet, insanely obvious ideas we somehow missed, not-so-obvious ideas we may be excused from having missed?"
And then: they watch and wait.
The computer calculates the fairy's velocity to estimate when they will be passing somewhere the trap can reach them. Mortal activates the automatic trap, so it'll shoot as soon as a fairy flies within range. He restricts the time window where it's active to the ninety-five percent credibility interval to minimise the risk of capturing some other unlucky fairy that may pass through (even though that's pretty unlikely), and then the trap is ready.
And should that not be enough, a tranquiliser dart will be shot at that fairy, and an invisible, inaudible, and warded-to-the-best-of-Promise's-skill Yellow should be heading that way to collect her.
Mortal injects her with something meant to wake her up, and that same first someone's voice comes from a speaker, still repeatedly saying "stop."
Mortal sends a message on the computer and the speakers says "Say your name."
"You may breathe. Answer all my questions truthfully and completely, using your sincere best up-to-date model of what I will find most relevant to prioritise. If you ever find any loopholes in my orders, inform me of them at once and do not attempt to exploit them."
"Except for genuinely personal information, what are you hoping I won't ask or think about?"
"How long would it take you to explain to me everything you think I would want to know about those things, and when are you expected at your destination?"
"How long would you normally take to arrive there from the spot where I captured you?"