The Further Adventures of Sherlock Holmes: The Devil's Promise

Sherlock Holmes and Doctor Watson are holidaying on the English coast when they discover a mysterious corpse on the beach. Then they are viciously attacked, and when Watson comes to, he discovers that months have passed, and Holmes is not the man he remembers. What has happened to his friend? Does it have something to do with the legend of a devil worshiper, who seeks only to raise his master?