The novel takes place in 1920. Maxwell Cheyne, a First World War naval veteran and short story writer from Dartmouth, is lunching at a Plymouth hotel, where he is doped and his home burgled. A few weeks later, a gang, searching for a vital letter a friend sent to Cheyne several years ago, tricks him onto a ship and holds him without food or water. After Cheyne hands over information about the letter, they put him ashore and threaten him to take no further interest in the matter. Instead, he pursues them, boarding a train into London. He follows them to a house in the Metroland suburb of Wembley, where he tries to reclaim the letter's contents. However, the gang strikes him on the head while he tries to steal it. Joan Merrill, a young female artist from Chelsea, rescues him and transports him to the hospital. In search of excitement, he agrees to join his mission to recover the stolen letter, which appears to contain some kind of ciphered code.

Only when the gang kidnaps Joan and attempts to kill him with an explosive device, which he narrowly escapes from at Marylebone Station, does Cheyne finally turn to Scotland Yard. Fortunately, Inspector French, with his understanding and thoroughness, takes on the case. Soon, his investigations lead him to Bruges and Antwerp, where he uncovers that the gang has recently embarked on an expedition associated with the stolen letter. The French now have to figure out exactly what the coded cipher means, as well as where the villains have taken the kidnapped woman. The key to the whole story seems to be a dead German U-boat captain and the secret he passed on.