Blunt's detective agency is doing well and Tommy considers the need for a larger office, in part to accommodate the shelf-space needed if they are to store the "classics" of Edgar Wallace if they are to copy his methods of detection. Inspector Marriot calls on the two sleuths with his latest mission for them; a large number of well-forged one-pound notes are in circulation and he wants them to track down the source. The West End seems to be the starting point for most of the notes that have been found (although some have come from across the channel) and the police are especially interested in the activities of a Major Laidlaw who is involved in racing circles and he and his French wife always seem to have a lot of money. Although it could be a coincidence, a large number of the notes emanated from a gambling club used by the Laidlaws and this, together with the racing, could be an ideal way of distributing the forgeries. Another friend of the Laidlaws is Lawrence St. Vincent (from A Pot of Tea) and Marriot thinks he could introduce the Beresfords into the set.
Marriot leaves and Tommy and Tuppence make their plans to catch the head of the forgers, or “The Crackler” as Tommy calls him, named after the sound that a rustled banknote makes.
The two are soon ensconced within the Laidlaw's circle of friends. As well as the Laidlaw's themselves, Mrs. Laidlaw's French father, M. Heroulade, is an object of suspicion. They observe how notes are passed by the Laidlaws to lay their bets and in among the wad of notes, there always seem to be some forgeries. Marguerite Laidlaw is a striking woman and has a string of admirers. Among them is a visiting wealthy American called Hank Ryder who tells Tommy that she is in fear of her husband. He's also been a victim of the forged notes as he tried to pay some into his bank that he'd received from his winnings and they were rejected over the counter.
The next night Tommy is at the gambling club that Marriot mentioned and Mrs. Laidlaw passes him several notes to exchange for one of a higher denomination. Among them are several forgeries. His immediate suspicions are directed to M. Heroulade but his attention is caught when he leaves the club and finds Hank Ryder drunk in the street outside. Within his slurred ramblings he tells Tommy how Mrs. Laidlaw took him on a treasure hunt which included a visit to Whitechapel where she "found" five hundred pounds. Tommy takes Ryder to the district and the exact house where they went to earlier. As the row of mean terraced dwellings look identical, Tommy chalks a small cross at the base of the back door before they enter. Ryder thinks he hears someone coming and goes back out to investigate. Tommy goes further into the house and finds the counterfeiting gang and The Crackler himself – Hank Ryder. He tells the captured Tommy that he has been suspicious of him from the start and that in case the chalk mark on the back door was a lead to Tommy's accomplices as to his whereabouts he has put similar marks on all the other doors. His satisfaction is cut short when Marriot and the police burst into the room and arrest the gang. Tommy tells Ryder that he was equally suspicious of him and so, when he was chalking the door, he also emptied a bottle of valerian on the ground, thus attracting all the neighbourhood cats to the smell. This was his pre-arranged sign for Albert who, on his orders, followed them to Whitechapel.