There are two separate stories going on here, Kivrin's story which is taking place in the past, and Mr. Dunworthy's story which is taking place in the "present" which is actually the future from our point of view. Kivrin has travelled back to the 1300's in order to experience a Medieval Christmas (the number of holy days means it will be easy for her to figure out exactly when she has arrived since time travel is usually off by a few days). Mr. Dunworthy is spending Christmas in Oxford, in the middle of an epidemic which begins very shortly after Kivrin leaves for the past.
As usual there are mysteries to unravel. The tech in charge of Kivrin's trip to the past is too ill to explain exactly what has gone wrong, the source of the epidemic is unknown, Kivrin fell ill shortly after arriving in the past and doesn't know exactly where her drop is located - vital information if she is going to be able to get back there in order to get home. The thing which struck me reading this was the amount of time the time travellers spend obsessing about how they will get back home again. It does make sense, being stuck in the past is scary, but they often seem to spend more time worrying about the details of exactly how they are getting home than in observing their surroundings. It certainly keeps the level of suspense high, but it is a bit disconcerting.
The big thing that struck me upon reading this is the subtle differences in how time travel works between the books. In this book the tech needs to establish a "fix" on the person who has travelled back in time that will allow them to reopen the "net" to the precise physical and temporal location in order to retrieve them. At one point in the story it appears that this fix has been lost (due to someone shutting off the power!) and so it will be impossible to retrieve Kivrin. In Blackout on the other hand, the time travellers have the expectation that failure to return on schedule via their original drop will result in a retrieval team being sent after them. This is time travel after all...if the folks in the future realize that there is a giant problem, they can just send someone back to the moment where the original traveller arrived, and have them return immediately. It does make sense that the mechanics are slightly different from book to book - it makes the plot work properly. If someone could have simply stepped through and prevented Kivrin's trip, the story would not have taken place. And the whole idea of sending a retrieval team probably didn't occur to anyone until afterwards. Still, it really seems like they might have come up with a better plan than "show up in this exact spot precisely two weeks after you arrived".
While the specific details of the time travel mechanics and protocols are in flux, which does sort of make sense as the technology is still relatively new, the story itself is fantastic and the characters are wonderful. I highly recommend it, but if you've got a choice I think they will work much better if you read Doomsday Book first!