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—
He sits up abruptly and gets up and marches towards the main room. "Thanks," he calls belatedly.
Gets up and goes to the bathroom to wash up and clean his teeth, it's not that he needs to be presentable but at least not having morning breath would be nice to Promise.
Then he takes his seat again and watches the screen.
He disassembles it neatly and orders a sorcerer to heal the mortal and fetch her out.
Promise is hovering over the button but she doesn't have to press it.
And she is fetched, healed and looking... Well, like she has not been having a fun time.
"Here- can she? Can you? None of them has been here—" He seems to be having trouble forming full sentences.
"An accurate description will do it but - we don't know what kind of psychological condition she's in and may not want her near anything sensitive."
"...oh. We can ask Thorn to rescind all her orders and... order her something else? Like no pressing any buttons?"
"Yeah. Can—I don't know if I wanna talk to her before she comes here and tell her it's—did Thorn do psychological torture? Like, could she believe it was all some ruse or game or something?"
"...he did psychological stuff but not in a disconnection from reality way. She should probably believe you're real."
"Okay. Erm. But then Thorn won't be able to hear our orders, we need to order him to—wait? Until I'm done talking to Mum?"
And she stabilizes Thorn and his vassals and has him give Laura his earbud.
"Mum! It's—it's me. You're—" He was about to say 'okay' but... "You're safe now. A friend and I—" Friend? Is Promise a friend? "We rescued you. We used a lot of those things you and I talked about. And we did it! We got Thorn, we stopped him, he'll never hurt you again, his name is Syracerix, he can't hurt you anymore now."