The story opens thusly: "At first he isn't even scared." And you're right, whoever "he" is, he's in deep trouble. In fact, he's about to die in a raging house fire. He was an old college friend of Ruth's, and shortly after she hears of his death, like, the next day, she gets a letter from him. He's been a professor of archaeology at a regional college in Lancashire, and he wrote to tell her he's made a tremendous discovery, but it's put his life in danger. Well, of course she hies herself and Kate up there to find out what happened. Unbeknownst to her, Nelson is wending that way, too, with his wife, for their annual visit at his mother-in-law's. Naturally - since they talk occasionally on the phone about Kate, and because Ruth calls him with her suspicions about her friend's "accidental" death, they end up investigating the situation together.
Several more of Ruth's old college gang appear in the story, and none of them are exactly what they seem. Nor are any of the college academics and students. Happily, for me anyway, she's brought her Druid friend Cathbad along to take care of Kate while she sleuths. Every story's better with Cathbad. (IMO)
The setting, around Blackpool, Preston and Fleetwood, are flavorfully rendered in shades of scuzziness, industrial bleak, and just for variety, glorious rural landscapes. The title of the book refers to the climax, when an amusement park roller-coaster provides some of the most suspenseful and dismaying moments in this series yet. I actually cried at the denouement - but won't explain, as that would be a spoiler.
Once again, Griffiths hits it out of the park and I again highly recommend this series!