the aliens are called Reshth - they are telekinetic shapeshifters.  They can only take humanoid shape (no animals), and they can only shapeshift by first having skin-to-skin contact with the living breathing person they wish to emulate.  By this contact, they also learn everything the person knows (including language...).  The victim is most often killed by the Reshth shapeshifter afterwards, as it is the most effective way to make sure you're not seen in two places at the same time...
The Reshth are invading Joth because their own planet is lacking in a great many resources due to extensive radiation.  Animals, plants, and people are dying; women are becoming sterile, and those that can still give birth are having skewed amounts of male children.  Females are becoming scarce themselves.  

First, a Reshth scout finds Chief Minister Zuun, kidnaps him, Touches him, and then kills him.  He allows himself to be "found" and warns of a possible alien invasion.  He is a little put-out by the fact that the King is on a two-week vacation (the Chief Minister thinks of Earth as simply "the King's other home", so the Reshth thinks the King has a summer home somewhere or something similar), but is willing to bide his time.  In the meantime, he sends word back to the Reshth army of what he has managed to accomplish thus far.  The King may not be here, but he (the Reshth) is in the next best level of power, and the only one on the throne at the moment is the king's young, fertile daughter.  (yes, it sounds like a bad porno plot (which Kaitlyn will probably remark upon at one point), but otherwise, they'd just kill her and take over and that'd be the end of the story...)
The Reshth army invades from the Jintaran border to the East, and "Zuun" convinces the Council to send only a small force comprised of their best men.  The Reshth army being larger, they soundly trounce the small army sent before them, ensuring the death of the best military men Zanorre has to offer.  
The Council wants to send the rest of their army after the invaders, but the Chief Minister again convinces them that the best thing would be to talk to the invaders and see what they want.  An invitation is extended, and of course, accepted.  And once inside the palace, the Reshth attack the Council.  (Ilexia was supposed to be there, but her mother has just been kidnapped at this point; she's on Earth trying to deal with that.)  The Ministers of Foreign Affairs and of the People manage to kill the Reshth trying to Touch them and run, hiding in the shadows to watch what happens to their counterparts (who are of course, killed after being Touched).  They decide it is in their best interest to go underground and try to figure out what to do before they're found out again.
Kaitlyn comes back and calls a Council meeting.  She asks about the conspicuous absence of the two missing Ministers, but is told only that they must be sick - their whereabouts are unknown.  She tells the Council that the Queen has been kidnapped, but that she will eventually be found and for now to keep it under their hats so as not to worry the public.  She asks how the battle against the invaders is going and is told that it is over.  She closes the meeting and says she's going to find the missing Ministers now and demand explanations.  She is told by the other Ministers that they are most eager to find the two of them.
She talks to Evyn about trying to find the missing Ministers, and Evyn promises to do her best, but says that there's been some funny things happening with the Council lately, and she's not sure what to make of it.  She says she'll keep an eye on them as well.  Kaitlyn goes back home to check on things with her mother; not much has changed.  
When she returns after a day or so, Evyn tells her that the Ministers are nowhere to be found, and that some strange new scullery maid has shown up who seems to know nothing about her work.  The palace staff are always impeccably trained before being hired, and this "maid" doesn't carry herself like a servant.  
The two girls corner the new scullery maid, who, in great fear, reveals that she is Reshth.  She stowed away on the ship when it came here because, at the time she left her home planet, she was about to become fertile (Reshth females only become fertile at a certain age, and stay fertile for the rest of their lives, as opposed to Joth and Earth women).  Her father was going to send her off to a breeding center - sort of a glorified brothel - and she fled rather than face having to have sex with hundreds of men.  She begs them not to send her back, and in return tells them everything she knows about her race.  She doesn't know their plans for Zanorre, however her knowledge helps greatly.  They promise not to turn her in.
Evyn goes to the person in charge of palace staff and tells them to take care of Kass (the Reshth girl) and to teach her how to be the best maid in creation (which includes the servant's art of being invisible) so that no one will suspect her.  Being the clever girl she is, Evyn has put two and two together and warns Ilexia that the Council (except for its two still AWOL Ministers) must be Reshth now.
Meanwhile, the "Council" has been busy.  They have declared a new groom for Ilexia: the Chief Minister.  Since Lord Zuun is OF COURSE from the royal family, he is a more than suitable groom for the princess.  With all of this Alien business going on, a great number of the people support the idea of having this "otherwordly" princess safely married off to a good solid Zanorran man.  Naturally Ilexia and her friends are terrified (esp. Quin).  She refuses the betrothal, but without her father around to back her up, there's very little she can do on this front.
She wants to go home and retrieve her father, but Evyn warns her that that would not be the smart thing to do.  Right now, he's in the safest possible place, and besides, she's not doing anything her father wouldn't do in her position.  Go to him to ask him for advice, fine, but he MUST stay on Earth!  Once on Joth, nothing would stop the Reshth from finding him, Touching him, and taking his place.
Of course, all of this marriage to Zuun business does NOT sit well with the twins, especially  love-struck Quin.  He goes to the Chief Minister and DEMANDS to know what's going on.  The confrontation gets violent, nearly ending in a battle, and Quin is arrested.  By this time, the Minister of Security has replaced all the higher-up palace guards with his own Reshth men, and the lower guards usually just do as they're told anyway. So no one questions taking Lord Quin into custody.
Once there, of course, a Reshth Touches him, in order to replace him.  He finds out from Touching Quin that Quin is in love with Ilexia, and that he has always been her best friend.  They decide to keep Quin alive but imprisoned for the time being; if Ilexia ever suspects that the "Quin" she's hanging out with is not the real one, they can still use the real one as a bargaining chip against her.  They beat the tied-down Quin within an inch of his life and leave him in the palace dungeons.
So Ilexia herself is walking on eggshells right now, and although becoming more and more depressed, she's putting up a brave front.  She runs into Herig who asks if she's seen Quin.  The two go looking for him and there is much relief when he is found safe and sound.  Ilexia's glad to see her best friend, but Herig seems suddenly concerned.  He had tried to talk to Quin mentally and had received no answer, not like Quin was ignoring him but like Quin couldn't respond (his mind automatically reaches for the real Quin's mind, and since he's currently unconcious from his beating...).  
Ilexia talks with her two best friends, telling them about her mother being kidnapped, and these alien invaders...what is she going to do about all of this?  Herig tells her she'll think of something, and Quin says that with him next to her, she need never worry.  She arches a peculiar eyebrow at him, but thanks him for the comfort.  (Quin would never have said that - he always talked about the two of them (himself and Herig) as being there for her and would never have left his twin out.)  
Of course, this means the Reshth know that she knows about them and their shapechanging abilities.  She knows the Council lied to her, which means she'll suspect them.
And indeed, she calls a Council meeting and demands to know why they kept all of this from her.  They cite concern about worrying her, and a few voice concerns that she is not yet equal to such a task.  Perhaps her father could be called home...?
She refuses.  Her father is dealing with the kidnapped Queen, and he has left her in charge.  She speaks with his voice, and anything they would tell him, they should tell her.  No one says anything to this.  So basically the Council knows that Ilexia knows that they aren't really the Council, but Ilexia doesn't know that the Council knows she knows (did you get all that?).  So there's an awful lot of frontin' goin' on.  Ilexia isn't about to call them out on it because it's 5 versus 1, and those aren't the best of odds.  And as long as she isn't about to stop them, why should the fake Reshth Council do anything any differently?
That night, Quin comes to Ilexia's room and says that it's far too dangerous for her to stay here.  He is her Knight-in-Service, and he must protect her; he suggests leaving the palace now, under cover of night.  She agrees and wants to pack; he tells her to hurry.  The plan REALLY is to take her to the Reshth mothership; with her safely imprisoned, a Reshth replacement Ilexia can be put on the throne until her father gets back.  All it would take would be one daughter-ly hug of dear-old-dad, and their plans are complete.  Meanwhile, they'd have the princess and could safely transport her back to a breeding center, no problem.  
Meanwhile, the real Quin has come around and is straining against his bonds.  Muzko, his pet royalcat, finds him and basically claws and smashes a hole in the older brick.  He runs in and bites the ropes holding Quin, who orders his pet to find Ilexia, NOW!  The cat takes off and Quin is as hot on his heels as he can be.
Meanwhile meanwhile, the fake Quin is informing Ilexia that while yes, it is his duty to protect her, there is another reason.  When she asks what it is, he tells her that he loves her.  Her heart skips a beat, and she's starry-eyed with romance.  She smiles at him, and he pours his heart out to her (all the better for her not to question where he's leading her).  "I've loved you for so long and been so afraid of telling you," blah blah blah - all the truth, from Quin's point of view.  
Just when Ilexia and (the guy she thinks is) Quin are about to share Love's First Kiss (soooo mushy - blech, remind me not to call it that in the actual story), Muzko bounds in and ATTACKS Quin.  Which of course, amazes Ilexia to no end.  And as she's trying in vain to pull the royalcat off of him, the real Quin shows up.
Well there's none of that "What the...? What's going on? TWO Quins?" sort of thing going on.  Ilexia just KNOWS.  She gets it now, and she is PISSED.
Quin and Muzko capture the Reshth invader, who still looks like Quin, and the real Lord Quin, Knight-in-Service to the Princess, interrogates his lookalike.  At first uncooperative, after several nasty royalcat scratches to certain especially sensitive areas of the body, the Reshth tells them everything that's gone on in the past two weeks.  Quin decides that they've gotten enough information out of him and draws his sword (after tying up the impostor Quin, he went and got it from his room).  Ilexia asks if it's really necessary to kill him, and tries to get him to at least not look like Quin.  Instead he looks at her and says, "Everything I told you was the truth, by the way.  It's just not me."  
Ilexia double blinks and Quin runs him through.  She shrieks a little and looks away too late.  Quin doesn't seem to have any trouble killing someone who looks just like him, although he does confess later that it was kind of...weird.  He asks her if she's okay, and she nods a little.
Quin drags the dead Reshth (who, in death, reverted to his natural form) out, telling Ilexia that he'll be back in a little bit.  He leaves Muzko with her so that if another Reshth shows up to try anything funny, she'll know immediately this time.  He leaves, and Ilexia is left to ponder the two new sides of Quin she has learned about tonight: (1) the Quin who could quickly, coldly kill an enemy and (2) the Quin who (maybe, might, if the Reshth really wasn't lying) is in love with her.  She doesn't love Quin any less for the fact that he killed someone, she's just a little squeamish about having seen it, and having the enemy look like Quin at the time.  This is probably the first time she's realized that other people live in two worlds also, but that theirs are just less literal: there's the face they show everyone and then there's how they really are.
Quin returns, and Muzko excitedly confirms that this is indeed the real Quin.  He says he's glad he and Muzko got here when they did, and she thanks them for coming to her rescue.  Quin says Herig thinks they probably won't try anything else tonight; it'll be a little while before they figure out their plan didn't work out anyway.  (Naturally, he's informed Herig through their telepathic bond of all that's happened.)  Still, he says he's going to stand watch over her door.  She refuses, telling him he needs his sleep, and she's a big girl, she can take care of herself.  Muzko yowls a little for attention, and they decide to leave the royalcat here, just in case.  Quin hugs Ilexia, grateful she's alright, and she snuggles against him and holds on tight to the Quin she knows for just a little bit, before letting him head back to his own room for the night.
The next morning, Ilexia is a little disoriented still by what went on the night before but she has had enough.  She calls in Evyn, verifies it's really her by Muzko's reaction, and tells her to find out if there are any of the Royal Guard that aren't Reshth.  If so, she wants them to meet in the stables at 10:00 am.  Evyn whines about only having two hours but hustles off to find them.  Ilexia goes straight to Herig with Muzko next to her.  She tells him her plan and he agrees with her but asks if she'd be so kind as to come back and talk with him after she talks to whatever remains of the palace guards.  She looks at him curiously and agrees, then leaves.  She heads back to Earth and catches her father up on things, telling/asking him about her plan.  He smiles at her a little sadly (still haven't found her mother) and gives his consent for her to go ahead with things.
So she goes back to Zanorre and to the stables, as it's almost 10.  The remaining Royal Guards who are definitely not Reshth (Kass can tell - she, Evyn, Quin and Herig are all there also) number only 20 or so, but that's more than enough for what Ilexia has in mind.
She tells them what's going on, then dismisses them to their positions.  Evyn goes off to inform the Council that there'll be an emergency Council meeting in an hour, and Quin and Herig take Ilexia aside.
Herig takes Ilexia to a hidden passageway, with stairs descending.  As they go down innumerable flights, he asks her what she knows of mechanica, that is, magic-powered machines.  Her knowledge is textbook, which is to say, as complete as any normally well-educated person's would be.  
The stairs end in a cavernous laboratory underneath the palace, and a huge mechanica hulks there before her.  Quin gathers up a strange sort of piloting suit and disappears into a curtained alcove off to one side, of which there are three or four.  Herig tells her to walk up to it, to touch it.  She looks at him curiously but does so.
As soon as her hand touches the metal, it begins to hum a little.  She squeaks and pulls her hand away and Herig fills her in on the fact that she is the power source for this, the Guardian Mechanica of Zanorre.  She's a little weirded out by this as Herig tells her everything about this mechanica.  He hands her a special connector suit, to amplify her power output and points to one of the curtained alcoves, telling her to change.  She walks in, changes, and comes back out to find Quin there waiting for her.  Both of them are in nearly skin-tight suits, and a big blushing fit ensues.  She asks why he's wearing a suit, and he says he's her pilot.  Herig chimes in with the fact that while Quin is piloting, their souls might merge a bit.  Another blushing fit.  Ilexia climbs into the center of the Guardian while Quin gets in up top and settles into the pilot seat.
Almost immediately the Mechanica begins to hum, and under Quin's direction, stands upright and walks around a little.  Inside, Quin and Ilexia are getting used to feeling so close to one another, even though they aren't physically.  Quin reassures Ilexia that she's in good hands; he can feel her nervousness. She says she knows she's in good hands...and strong arms...her mind begins to wander...beautiful eyes...
The sudden wave of embarassingly frank love that hits them both shuts the mechanica down suddenly, as Ilexia tries to pull her mind away from Quin's.  Herig interrogates them as they climb out - it was going so well, what happened?  Neither feels like answering, and they're having trouble meeting each other's gazes.