Inspector Marriot arrive at the Blunt’s detective agency for a new case involving rounding up a seemingly big gang involved in circulation of counterfeit one pound notes. He hand Tommy a counterfeit note to examine the difference with the real one. With Tommy unable to figure the difference, Marriot trains them to spot the counterfeit notes. He asks them to get to the to bottom of the matter and nab the culprits. The notes, Marriot informs them, are being circulated from West End according to their discovery, and someone high in the social ladder is involved. They also pass these notes to other side of the channel as well. The prime suspect in the case is Major Laidlaw, who is probably involved in few shady transactions. Not much is known of his past, he has a French wife, and the investigators are interested in knowing their source of money.
Marriot further informs that a lot of fake notes come from a gambling club and racing circles frequented by Laidlaws. He suspects that they get rid of fake notes using these mediums. To get acquaintance of Laidlaws, Marriot asks them to take help of Young St Vincent. Marriot also asks them to keep an eye on Mrs. Laidlaw’s father M.Heroulade since the notes are being passed on both sides of the channel. Tommy names the people involved in counterfeit note circulation as Cracklers, based on the sound produced by banknotes.
Tommy and Tuppence easily makes acquaintance of the Laidlaws. Tuppence gets ten one pound notes from Jimmy Faulkener, who got it from Margeurite Laidlaw for placing a bet on a racing horse. She exchanged for a tenner. On examining Tommy confirmed that they are actually fake notes, further strengthening their suspicions against Laidwaws. Hank Ryder is a wealthy man in the circle of Mrs. Laidlaw. From Hank, Tommy fetched more information about the bets place by Mrs Laidlaw, and fake notes rejected by his bank.
Next day at the Gambling Club, Tommy gets more evidence of Mrs. Laidlaw’s involvement when many small denomination notes she exchanged with Tommy in return of a bigger note were found fake. Tommy also begins the investigation of source of counterfeit note supply to Mrs Laidlaw. He eliminated Mr Laidlaw from the suspects with the cooperation from Albert who tracked his movements. So the needle of suspicion pointed towards Mrs Laidlaw’s father M. Heroulade who often moved to and fro to France.
Tommy is trapped
Exiting the Gambling club, Tommy finds Mr. Ryder drunk and blabbering random stuff. Moments later, Ryder attained some command over his speech and told Tommy about a place where Mrs Margeurite took him. Suspecting that the place where she took Ryder might be the headquarter of generation of fake notes, he reaches the place with the assistance of Ryder. Without much ado, Ryder identifies the house where Mrs Laidlaw went to fetch the notes. Since all the houses look same, Tommy made a mark using chalk at the door of the concerned house. There were many cats in the alley, Tommy discovers. The door yields easily and they enter the flat. Soon Tommy discovers that he is trapped and Ryder is the actual Crackler.
Marriot soon arrives and arrests Ryder and his gang. It turns out that Ryder was key suspect from the beginning. Albert followed Tommy and Ryder ,as per the instructions imparted to him. While marking the door with chalk, Tommy emptied full bottle of Valerian on the ground. It attracted cats and exposed the exact location of the house to Albert who communicated it to Marriot.