The Great Mouse Detective is adapted from the children's book series Basil of Baker Street by Eve Titus, which translates the Sherlock Holmes world into a children's setting featuring mice in all of the leads. The movie was delivered in 1986, right before Disney's big renaissance period was ushered in with The Little Mermaid. It's interesting bit of trivia is that it was the first major animated feature to use computer graphics (which can be found in the climactic clock-tower scene). The film features the voices of Vincent Price, Barrie Ingham, and Val Bettin in the lead roles.
The roles will be familiar, but the names have been changed. In The Great Mouse Detective, Sherlock's mouse counterpart is Basil of Baker Street, and his trusty companion is Dr. Dawson. The duo are trying to solve a case involving Basil's arch-nemesis, the devious and treacherous Ratigan, "the world's greatest criminal mind." This particular tale finds Bail and Dawson helping a young girl by the name of Olivia Flaversham, whose father - a skilled toymaker - has been abducted by Ratigan. But why? That's what the duo must find out. And all they know is that it seems to coincide with the Queen's sixtieth gala celebration.
The Great Mouse Detective comes near the end of Disney's run of making films that weren't adapted from fairy tales. Some of the stories skewed older, many of them became less formed around the structure of a musical, and some of the "formula" for a Disney film was stretched. One could argue that reversing all of these trends is what made The Little Mermaid a return to their earlier successes: it fit the well-established success mold.
But all of this is part of why The Great Mouse Detective feels somewhat unique in the Disney timeline, because it's decidedly non-Disney in comparison to many of the films that surround it. The first difference is the tone, which stays rather dark throughout. Literally, in fact, as there isn't a single scene that takes place during the daytime. The stormy night setting and constant lurking in shadows keep the film from having more than cursory light moments. The overall feel of the film is somewhere between a style comfortable to the studio, as well as borrowing a note or two from The Secret Of NIMH.