We do love a man in a uniform, but the "Constables" in question are not policemen but paintings-the landscapes, specifically, of the 19th-century painter John Constable. Agatha Troy (the artist wife, you'll remember, of Inspector Alleyn) has a special fondness for Constable's work, so she jumps at the chance to take a river-cruise through "Constable Country" in the east of England. Her enthusiasm dims a little when it becomes clear that the ticket became available at the last minute only because a previous passenger was murdered in his cabin-and murdered, it seems, by a notorious international criminal known as the "Jampot." (How we long for the days when notorious international criminals had really cute names.)