Shoal Water

''Shoal Water'' (1940) is a thriller by British author Dornford Yates and belongs to the author's collection of "adventure thrillers, " often categorized alongside the Chandos series, though it features a different primary narrator. The story is narrated by Jeremy Solon, a young man who falls in love with Katharine Scrope (also known as "Formosa"). Katharine has been blackmailed into serving as a courier for a jewel-thieving gang led by a villain known as "The Shepherd". When Solon becomes a target after identifying the gang's leader, Katharine is kidnapped.
To rescue her, Solon joins forces with his friend George Laking and Yates's recurring hero, Jonathan Mansel. The action takes place across France, notably featuring a pivotal location called "The Wet Flag" and the waterfront of Rouen off the English channel.