The story revolves around the great mouse detective in question, Basil (a play on the name of famous Holmes actor Basil Rathbone), and his new-found friend, an ex-army surgeon just returning from Afghanistan, Major Dr David Q. Dawson (based on Watson). The story concerns Basil hunting down his nemesis, Ratigan, who is loosely based off Moriarty. Basil is approached by a little girl named Olivia, whose father has just been kidnapped by one of Ratigan's henchmen. Together they end up solving the case, and saving queen and country as well. In the end, Ratigan meets his own death while trying to kill Basil.
There are many similarities, as well as differences, between this film and the original Sherlock Holmes stories.
Similarities to the Sherlock Holmes stories
- Dawson is an army surgeon returning from Afghanistan (As was Watson).
- Basil dresses like the stereotyped Holmes did.
- Basil lives in relatively the same place as Holmes (Actually under the 'real' one, as it were).
- Basil has hobbies that include chemistry and playing the violin.
- Basil has a knack for disguise.
- There is a landlady, named Mrs. Judson, who is very similar to Mrs Hudson.
- They make use of a dog named Toby. A dog named Toby was used in "The Sign of Four."
- Basil has mood-swings, going from languid to eager in an instant, much like Holmes.
Differences from the Sherlock Holmes stories
- Ratigan is shown as a rather different character than he was in the books as Moriarty. He is shown as intelligent, but much more outlandish and is shown with a lot of brute physical strength, which was never the case in the canon.
- Basil and Ratigan are arch-nemeses. This Holmes vs. Moriarty hatred is reoccurring in many spin-offs, but it is arguably wrong, seeing as Moriarty was only present in a few books, and Holmes respected his intelligence, rather than hating him.
- Basil is forgetful (Mainly of Olivia's last name, Flaversham), which Holmes never was.
- Obviously as the film was intended for children, the subject matter is much lighter than it was in the original, which often focused on solving murder-mysteries.
- Based on the book Basil of Baker Street by Eve Titus.
- At one point, a shot of the 'upstairs', 221B Baker Street, is shown in silhouette. You can hear Holmes and Watson discussing a case. The voice for Holmes is actually the voice of Basil Rathbone, even though he died 20 years before the movie was made. His voice was overdubbed and added into the movie from a reading of The Adventure of the Red-Headed League.
- Basil Rathbone as Sherlock Holmes: The famous human detective who lives above Basil. His voice is taken from clips of the original Sherlock Holmes films.
- Laurie Main as Dr. Watson: The medical associate/partner of Sherlock Holmes, who also lives above Basil. Unlike Rathbone, voice clips of Nigel Bruce were not used for the cameo part of Watson.
- Barrie Ingham as Basil: A detective, based on the fictional sleuth Sherlock Holmes and the primary protagonist of the film. His main goal is to get Professor Ratigan behind bars and rescue Olivia Flaversham's father, while simultaneously preventing a royal assassination. There are a few differences between Basil in the book series and in the film version, such as mercurial moods in the latter. He also plays the violin rather well in the movie, whereas the book series stated Basil's violin playing was atrocious— instead, Basil played the flute.
- Vincent Price as Professor Ratigan: Basil's arch enemy and the primary antagonist of the film. He is based on Professor James Moriarty from the Sherlock Holmes stories. He plots to seize control of the British monarchy. He and Basil are long-established arch enemies. In the book series, it is revealed his given name is Padraic and that Ratigan is, in fact, a mouse. At the end of the film, he is knocked off Big Ben and disappears into the deep chasm below, following a fierce battle with Basil, similar to the Sherlock Holmes story "The Final Problem".
- Val Bettin as Major Dr. David Q. Dawson, previously of the Queen's 66th Regiment in Afghanistan. His character is based upon Dr. John H. Watson from the Sherlock Holmes stories. The interaction between him and Basil mimics that of Watson and Holmes, as Dawson is constantly amazed by Basil's deductions. He eventually becomes Basil's associate, friend, and personal biographer.
- Susanne Pollatschek as Olivia Flaversham: A young female mouse of Scottish descent who seeks Basil's help in finding her toymaker father.
- Candy Candido as Fidget: Ratigan's bumbling bat henchman. He tends to do the dirty work for his boss. He has a crippled wing and a peg leg, and, as a result, he cannot fly. Ratigan throws him off the side of his flying machine near the end of the film, and he falls into the River Thames. It is revealed (though in the book, not in the movie) that he survived the fall (how is not explained). Perhaps he does not survive in the film.
- Frank Welker as Toby: Basil's trustful Basset Hound. He technically belongs to Sherlock Holmes, who lives above Basil.
- Alan Young as Hiram Flaversham: Olivia's loving Scottish father. He works as a toymaker, and is kidnapped by Fidget to make the Queen Mousetoria robot for the evil Ratigan.
- Frank Welker as Felicia: Ratigan's massive and pompous pet cat. Ratigan calls upon her by ringing a special bell to dispose of traitors or anyone who makes him angry. She is chased by Toby and attacked by Royal Guard Dogs in the final part of the movie and is not seen again. She is presumed dead.
- Diana Chesney as Mr. Judson: Basil's housekeeper. She adores Basil, but gets very annoyed when he mistreats her good pillows by shooting them with a pistol. She is based on Mrs. Hudson.
- Eve Brenner as Queen Mousetoria: The mouse queen of England, whom Ratigan attempts to depose of. She is a parody of Queen Victoria and the setting for this film coincides with the real Victoria's Diamond Jubilee (as can be seen with humans entering Buckingham Palace at the same time the mice are).
- Barrie Ingham as Bartholomew: One of Ratigan's henchmen. He meets his end early on during Ratigan's song when he drunkenly calls his boss a rat; enraged, Ratigan throws him outside and summons Felicia, who devours him.
- Jewish musician Melissa Manchester as Miss Kitty Mouse: A beautiful mouse who with her twin sisters are dancers at The Rat Trap, a seedy dive on London's waterfront.