This is the third and best Doctor Who novel I've read yet. J.T. Colgan didn't overdo the Doctor and she didn't manage to create a totally stupid monster. I enjoyed reading it, the characters were well developed and stayed true to their personalities, and it occasionally made me laugh. It helps that one of the main characters thinks the Doctor is Loki for the entire book.
It takes place on the tip of what will one day be Scotland, with lots of friendly people who aren't too fond of the vikings that attack them every so often. It also takes place on a boat full of vikings and one Nordic princess, who is being taken to her wedding to the revolting king of some place. But that's only until the boat is consumed in fire... while it's floating in the middle of the ocean. Most of the crew are killed, but those who are not- the Princess included- definitely want to know about the fire that burned on water, and about the mysterious man in a bow tie who rescued them.
But the fire doesn't stop at the water. It soon travels to people, using their minds until they burn up. The Doctor isn't sure what to do about all that, so he does the usual thing, talks to the alien and asks them what they think they're doing.
With the help of Princess Freydis and her unlikely boyfriend, the viking Henrik, The Doctor talks to the underwater-dwelling, shape-shifting alien called the Arill and, after a few more deaths, figures out how to defeat it. And everybody (except the people who died and their families) lives happily ever after.