Part of what happened is that a alien spacecraft has crash-landed near the village, and the main story involves the interactions between the aliens and the townsfolk.
It's a great story, and Flynn put a lot of effort into researching and portraying a vanished time and a vanished way of thinking about the world. But forget the research... it's just a good story. The modern-day portions are much less satisfying, due partially to some jiggery-pokery with physics that gave me the pip. Then again, maybe medieval historians would be given the pip by the part I liked.
I can't really say why, since there seems to be no reason for it, but I quite liked how certain terms seemed to be coming out of German translation. So, to get self-referential about it, the aliens have translators that produce 'oversetting' [i.e. Übersetzung in German]. More cutesy/less satisfying was the manner in which the village priest always chose a highly appropriate Greek root to form a 14th century 'neologism' for alien technology, coincidentally reproducing the modern term. Sure it happened in our universe, but it's hard to imagine getting from 'first' and 'amber' to protein and electron.
Overall, though, a smart work with well-drawn characters and an interesting story (at least the 14th century stuff, which makes up the bulk of the novel).