The book begins with two Oxford history students, Lewis and Simon, who drive to Scotland to look into a mysterious appearance of an aurochs and end up in the Otherworld. It's your basic, lost in another world plot.
Unfortunately this book is one of the poorer executions. The plot of the novel is paper thin, nothing more than a chain of the 'and then this happened' sort. Substantial amounts of text are used to describe how magical and wonderful everything is in the Otherworld, which is great if you're into that (but I'm not).
The PoV, Lewis, is dull. He doesn't initiate any action, only getting dragged around by circumstance and if the plot demands it. This ruins the little plot there is, making the book a slog to read as either nothing happens or we get another scene where Lewis is lamenting his situation.
Disappointing all around. Don't bother with it unless you really like ancient Celt stuff, because that is all the content this book has.