Explain the mystery that Jabez Wilson wants Sherlock Holmes to solve.
Jabez Wilson tells a long back-story concerning how he came to be engaged at the Red-Headed League and had been receiving four pounds a week for eight weeks for copying articles out of the Encyclopedia Britannica. Then that very day he came to the office and found a sign on the locked door stating that the Red-Headed League had been dissolved. Wilson tried to trace the man called Duncan Ross, but his forwarding address was false. Wilson would like very much to get his old job back if possible. Failing that, he would like to find out why someone had played this trick on him. As Wilson expresses it:
“No, sir. But I want to find out about them, and who they are, and what their object was in playing this prank—if it was a prank—upon me. It was a pretty expensive joke for them, for it cost them two and thirty pounds.”
Holmes only agrees to take the case because he sees a lot more implications in the avaricious pawnbroker's experience than Wilson ever suspected. Holmes later explains his deductions and suspicions to Watson after the infamous John Clay has been apprehended.
“You see, Watson,” he explained in the early hours of the morning as we sat over a glass of whisky and soda in Baker Street, “it was perfectly obvious from the first that the only possible object of this rather fantastic business of the advertisement of the League, and the copying of the Encyclopaedia, must be to get this not over-bright pawnbroker out of the way for a number of hours every day."
The Red-Headed League had been dissolved because the tunnel from Wilson's pawnshop to the bank had been finished and the crooks were ready to loot the French gold coins that night. They no longer needed to keep Wilson out of the way. No doubt the solution to the mystery was fully explained to Wilson either by Holmes or by the police, including the fact that his assistant was really John Clay and was going to be hanged.