American heiress Henrietta Doran (known as Hatty) marries Lord St Simon, but she vanishes without trace on her wedding day, leaving London agog. Later, the police find her wedding dress in the Serpentine, a lake in a London park, with a note in the pocket of the dress. While investigating the case, Holmes suffers from nightmares, which he sees as a factor in the mystery. (thetvdb.com)