Six passengers in a locked train carriage. One of them is dead. No one saw a thing.

Former Detective Hillary Greene is off on holiday. For a blissful two weeks, her life will be free of murder victims, murder suspects — and, best of all, no paperwork. She’s heading for the beautiful Welsh town of Hay-on-Wye, where she plans to spend her time browsing old bookshops and generally relaxing.

Hillary arrives at the picturesque Riverside Inn where the other guests are an eclectic a charming Irishman, a rich American and his spoilt daughter, a thrice-married Canadian influencer, a wealthy widow and a taciturn taxidermist.

The next day, Hillary and her fellow guests set off on a steam train ride through the stunning Cambrian Mountains. But when the train arrives at its final destination, not all the passengers get off.

One of them has been stabbed neatly and cleanly through the heart.

Which of the five passengers who shared the carriage is the killer? And how come no one saw a thing . . . ?