Now retired from detective work, Sherlock Holmes is called on once again to do battle with occult forces. Unrest in Europe is threatening to turn into all-out war, and there are certain parties who would seek to gain from the mass slaughter.
When his brother, Mycroft, is found dead on a lonely stretch of beach in Sussex, the body covered in slime and hideously mutilated, Holmes unearths a plot against members of the Diogenes Club that has far-reaching, apocalyptic consequences.
With trusty Watson once more at his side, Holmes sallies forth one last time to defy the nameless, unspeakable entities from beyond who would wish to conquer the earth and make every human their slave.