Summer, 1939: While Britain and France teeter on the brink of war with Nazi Germany, Griff Wilmot, a cavalry officer turned schoolmaster, discovers that his French wife Simone is having an affair with Lieutenant Commander Archie Trendle-Home, his closest friend. On the outbreak of war, with his home life crumbling, Griff goes back into uniform and takes command of a section of Sappers in France. Archie is given command of an elderly destroyer, and Simone finds herself acting as landlady to three Wren officers who are serving in the naval headquarters at Dover.

Alone and vulnerable, Simone receives an unsettling visit from her illegitimate son, David Odell, who has come to England from New York to seek out his natural mother.

When Germany invades the Low Countries in May 1940, Griff Wilmot's section is in the front line, and he and a host of factual and fictional characters are soon caught up in the retreat to the coast and the evacuations of Boulogne and Dunkirk.

No one who lived through those dark days emerged unscathed, stories of Simone, Griff, David, and Archie encapsulate those of thousands more.