Martin Overshaw and Corinna Hastings are leading dull and unproductive lives in Paris, having fled humdrum England. They fall in with Fortinbras, who calls himself a Marchand de Bonheur. He predicts a bright future for them and suggests they set out on a journey through France together. The book follows their adventure which turns out to be far more complicated than it might at first seem. They meet a variety of characters on the way and the looming threat of the First World War overshadows the second half of the book, which nonetheless ends happily for all concerned.