It's Christmas 2001. Lucy (because why not Lucy? Actually, I'd probably come up with a name that sounds like a valid Sindarin name, because it would lead to humour as she tries to explain that no, it's English...) has seen The Fellowship of the Ring three times already (and it's only been out a week!). Now her best friend has bought her the book it's based on. She's a slow reader, but that's okay - for this book, she'll make the effort. Even if it doesn't have (let's buck the Legoluster trend a little and say) Elijah Wood in it.
Then, due to unexpected and unexplained magic, Lucy is dragged into Middle-earth. In fact, she's dragged to Rivendell - though it doesn't look a lot like the film. Nor does the very tall and really kind of scary guy looming over her look much like Agent Elrond (she thought that was funny when her friend told her about it, but now - eek!).
She is, of course, totally unable to answer - or understand - and of Elrond's questions, but seeing as how she appeared in mid-air in the Hall of Fire, he's not inclined to do anything hasty about her (though, of course, she doesn't know this, and he does have scary eyebrows). And besides, there are other things on his mind... like the recent arrival of Mithrandir (have to stick to the Elvish to avoid translation issues, a la Don't Panic!, though fortunately I've since solved the Gimli problem), and his report that a certain Ring is on its way to Imladris...
So Lucy is given into the guardianship-and-care of... let's say Glorfindel and an OFC, Lingalad. Lingalad because a female prisoner-guest-mystery will probably respond better to a woman, and Glorfindel because as a Valinorean originally, he's had to learn at least three, probably more, languages (starting in Quenya, he would have learnt Sindarin in Beleriand, Adunaic in the Second Age, and Westron in the Third; he may also have learnt various Mannish tongues in any of those three); this makes him the best person in Imladris to try and figure out Lucy's words.
And so the time of Frodo's convalescence is taken up by Lucy learning very basic... Sindarin or Westron, probably the latter, since she's self-evidently human. And just as she's reached the point where she can at least make her needs and questions understood, and tell them very basically about herself (she's had to say she comes from 'above the sky', because she has no idea how to say 'another world'), she discovers that the Fellowship (several members of whom have been concerned or at least curious about her, and she's gotten to thinking that Frodo - beg pardon, Maura - is actually pretty cute, even if he doesn't have Elijah's eyes) are about to leave.
And she's seen the film.
And she knows what's going to happen.
And she still has the book with her.
With rather a lot of help from Glorfindel - who's better at English than Lucy is at Westron - she manages to explain to Elrond what she's got - she can prove it by reading out some things there's no way she could know, like the precise words spoken in private conversations. And she tells him that Gandalf is going to die, and so is Boromir. And, she explains, she hasn't had a chance to read the rest yet...
This decision goes to Elrond's state of mind. I'm inclined to say that, with the Ring Quest being the most important thing in the world right then, he'll send Lucy with the Fellowship rather than keeping her behind as a resource. He'd love to know what's in the rest of the book - enemy movements, threats to allies? - but ultimately, if Imladris falls and the Ring is destroyed, he will consider that a victory. So he sends her along - and urges her to read fast.
The Fellowship avoid Moria, of course, given that Lucy knows already that Gandalf will die and Gollum will start to follow them (which in her mind is unambiguously bad - remember, she knows nothing about anything after the Breaking of the Fellowship!). I don't know what route they'll take - perhaps Boromir swings the idea of the Gap of Rohan, or Legolas convinces them to try the High Pass. And Lucy proves moderately helpful. Let's send them through Rohan, where she can warn them in advance that things are not as nice right now as Boromir would like to think - and she's bright enough to turn to the timeline for the major battles.
And then she reaches the point where Gandalf comes back as the White, and realises how badly she's messed things up.
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