Time travel from the near future to the medieval past. . . .
Not very well-thought out travel. Kivrin, the student doing it, has made plentiful preparations, educating herself, digging at an archeological site to get her hands right, being inoculated, among other diseases, against the Black Death even though they're aiming for thirty years earlier, bleeding from a head injury to lend authority to her story, but Dunworthy angrily points that they aren't really prepared -- no unmanned drops to medieval times, after all.
She goes off. Badri, their tech, goes to get the fix on her, and she heads out with her story of having been attacked by robbers and her carefully chosen name of Isabelle. Dunworthy goes off with his friend Mary to a pub to avoid the Christmas preparations, and hears how her grand-nephew is coming to spend Christmas with her. Then Badri comes to find him, and collapsed, ill. Dunworthy's worries are a bit assuaged because Badri had said that slippage was minimal, and because he knew that if Kivrin had carried whatever Badri had, the net would have not opened, thus preventing her from infecting the past.
Meanwhile, in the medieval era, Kivrin is deathly ill with fever, which the people who find her attribute to her head wound. She's managed to convince them that her name is Katherine in her fever. Her translator does not work well for a time, but she starts to manage and finds herself in a noble household -- a woman, her mother-in-law, her daughters, with the father off in London about a trial.
With nothing going neatly, everything interweaves into the plot. It involves a tomb from the medieval era, a quarantine, ringing the bells for the dead, a betrothal between a 12-year-old girl and a man of at least fifty, modern bell-ringers carrying out changes, a lack of supplies, a hound puppy, barricades, a boy getting a muffler and a book about medieval history for Christmas, an over-bearing mother who reads dramatically condemning parts from the Bible, and much more.
I warn that the story is not light. I gave it to a cousin of mine one Christmas, and she had stayed up late, finishing it, and it is not perhaps the best book to finish reading in the wee hours of the morning in your dorm room.
It's an alternate history now, since they have beepers and no cell phones in the 21st century. (After a Pandemic which was apparently a traumatic experience all around.)