When a bored Sherlock eagerly takes the case of Gabrielle Valladon following an attempt on her life, the search for her missing husband leads to Loch Ness and the legendary monster.
| Actor | Role |
|---|---|
 Robert StephensSherlock Holmes | Sherlock Holmes |
 Colin BlakelyDr. John Watson | Dr. John Watson |
 Geneviève PageGabrielle Valadon | Gabrielle Valadon |
 Christopher LeeMycroft Holmes | Mycroft Holmes |
 Tamara ToumanovaMadame Petrova | Madame Petrova |
 Clive RevillNikolai Rogozhin | Nikolai Rogozhin |
 Irene HandlMrs. Hudson | Mrs. Hudson |
 Mollie MaureenQueen Victoria | Queen Victoria |
 Stanley HollowayGravedigger | Gravedigger |
 Catherine LaceyWoman in Wheelchair | Woman in Wheelchair |
 Peter MaddenVon Tirpitz | Von Tirpitz |
 Michael BalfourCabby | Cabby |
 James CopelandGuide | Guide |
 John GarrieFirst Carter | First Carter |
 Godfrey JamesSecond Carter | Second Carter |
| RRobert CawdronHotel Manager | Hotel Manager |
 Alex McCrindleBaggageman | Baggageman |
 Frank ThorntonPorter | Porter |
 Paul HansardMonk | Monk |
 George BensonInspector Lestrade | Inspector Lestrade |
 Anne BlakeMadame | Madame |