When McGurk gets sick and his doctor turns out to be a Sherlock Holmes fan, his fellow kid detectives have to humor their leader's new self-image. Not only does McGurk receive his friends in a silk dressing gown (a gift his father never wears) but the kids discover that those terrible sounds issuing from the convalescent's room are McGurk on a new second-hand violin, which he'll play to help him think about tough cases. And as cases do with the McGurk organization, a new one turns up in the form of a mysterious folded note inside the violin. Investigation tells the kids that the violin belonged to a jewel thief who died in jail, his last heist of uncut diamonds still missing. So it's only a matter of puzzling out the strange map on the note--it turns out to be a drawing of tree bark, with the felon's initials on the tree--and then puzzling out more notes in a treasure-hunt sequence that leads them to the jewels. Each Md contributes something to the solution; they have some unpleasant competition toward the end; and everything zips along at the usual entertaining pace.