The film begins in a theatre, where a pianist (Moore) begins to play a piano accompaniment to the actual film being shown in the theater.
Holmes (Cook) has just restored a stolen artifact to three French nuns, and is later called on a case by Dr. Mortimer (Terry-Thomas) concerning Sir Henry Baskerville (Kenneth Williams) and a legendary hound that curses the Baskerville estate. Tired and worn out by so many cases, Holmes passes the case onto Dr. Watson (Moore), who is portrayed as a Welsh eccentric.
Upon arriving at the station, Sir Henry, Dr. Mortimer, Watson and Perkins (their driver) are halted by a policeman (Spike Milligan), who warns them of a murderer stalking the moors, before sending the group on their way.
While out in the moor collecting specimens, Watson has a curious encounter in a hut with the raving Mr. Frankland (Hugh Griffith) and Frankland's exceedingly strong daughter, Mary (Dana Gillespie ).
In the next scene, we see Holmes still in London, visiting Ada, his mother (also played by Dudley Moore), who, as a bogus spiritualist aided by her housekeeper, Iris, scams older ladies of their money in false seances. Holmes' mother is concerned that "Watty" (Watson) may need help, and that "Sherl" (as she calls her son), should rush to his aid.
In next scene, Holmes interviews the one-legged Mr. Spiggot (also played by Dudley Moore) to act as a runner on the moor.
In the next scene, while wandering on the moors, Watson happens upon Mr. Stapleton (Denholm Elliott), who mistakes him for Sir Henry. Stapleton's sister (Joan Greenwood) describes dramatically her encounter with the Hound on the moor, suggesting that it ravished her, whereupon she attempts to force herself upon the reluctant Watson, with Miss Stapleton undergoing supernatural transformations reminiscent of The Exorcist.
The Barrymores (Max Wall and Irene Handl) at Baskerville Hall mistreat Sir Henry and Watson, feeding them only cheese and water and then throwing them into a small bedroom, ankle-deep in water. Watson then goes to the village to send a message to Holmes (who is during this time visiting his cranky mother), and meets Mr. Stapleton of Merripit Hall. Stapleton is carrying a chihuahua that proceeds to urinate in Watson's pocket and face.
Arriving at Merripit Hall, Watson meets the eccentric Mrs. Stapleton, who displays surreal symptoms suggesting demonic possession. Late at night, Sir Henry and Watson discover the Seldons and the escaped murderer, whom Watson recognizes as Mrs. Barrymore's brother Ethel Seldon (Roy Kinnear), having a family dinner. Oddly enough, neither of the men seems to panic at this.
Afterward, Holmes arrives and examines the case so far. An invitation arrives for Sir Henry, asking him to dinner at Merripit Hall. Suspecting a trap, Watson goes along with Sir Henry while Holmes observes carefully. Mrs. Stapleton resumes her bizarre acts and begins to vomit pale-blue liquid over Sir Henry, whilst Mr. Stapleton's chihuahua urinates in Watson's soup.
Ordered to leave in disgrace, the Stapletons, Dr. Mortimer, Mr. Frankland, and his daughter Mary follow Sir Henry and Watson to kill them, but become trapped in a quagmire. Holmes then proceeds to reveal that the Hound is no more than a large, rather friendly Irish wolfhound owned by the late Sir Charles Baskerville, whose excited barking was misinterpreted as a monstrous beast. He also states that the dog is the sole heir of Sir Charles. With the dog gone, the would-be murderers would have gained the Baskerville fortune and the estate.
The film ends on the pianist, who is then hit by vegetables from the audience.
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