Inspector Lestrade brings Sherlock Holmes and Doctor Watson an account of six murders. The only fact common to them all is that the victims had all served terms of imprisonment for serious crimes. It appears that someone believes that they should have suffered execution, and has set out to take the law into his own hands. A note is found, signed 'The Justice Master', warning that he will not be deterred from his purpose, nor does he spare anyone who gets in his way. The inspector is angry with his superiors because he feels that their efforts to apprehend this man have been insufficient, hence his consultation with The Great Detective. The murders continue and Holmes, with the occasional interference of an Old Bailey solicitor, Mr David Grantly-Knight, resolves to bring the killer to justice. The Justice Master proves to be as elusive as he is merciless, and narrowly escapes a trap set by Holmes and Lestrade. Then comes an encounter near Plantain Castle.