The second of four spy novels featuring a British agent named Harry Palmer, the novel takes place in 1960, mostly in a small fishing village in Portugal, during the Salazar dictatorship. As with several of Deighton's early novels, the chapter headings have a common theme. Here, these are crossword clues, reflecting the protagonist's habit of endlessly writing and replacing words in crossword puzzles. The plot centers on the recovery of objects from a German submarine that sank off the Portuguese coast in the final days of World War II. The items were originally counterfeit British and American currency to cheaply finance a revolution in Portugal. Later it's heroin (the "Horse" of the title), and eventually it turns out that the real interest is in the "Weiss List" - a list of Britons willing to help the Third Reich create a puppet government in Britain if Germany wins.