The villainous Dr Le Doux, previously an abortionist, has set up a new Sanatorium for nerve sufferers, having bought someone else’s name and qualifications. He provides the very latest in gadgetry for the comfort and treatment of his patients, notably the ‘ventilation method’, which is secretly operated from outside the room so that the nervous asthmatic patient is not alarmed by the use of oxygen. However, Le Doux has the makings of other, deadly, gases, and provides one of these in a ‘Purple Flask’ to the evil Miss Gwilt. Their plan is to trick the hero Armadale into staying, and then kill him.
Suffice it to say that the plan does not work out as expected, and Miss Gwilt is found dead, possibly from an ‘apoplexy’. The epilogue comprises a letter from the family solicitor, who suspects Dr Le Doux of a great deal, but nothing can be proved; indeed, his friends and admirers are about to present him with a Testimonial expressing sympathy.