- “But I shall always look back on that first with the most fondness: my introduction to Basil of Baker Street, the Great Mouse Detective.”
- ―Dr. David Q. Dawson, closing narration[src]
Basil is a take-off of the classic Sherlock Holmes character, modeling his career after the famous detective's. Basil is portrayed as a brilliant, plucky mouse who refuses to back down or end his pursuit against Prof. Ratigan, which makes him tenacious. Basil comes off as a jack-of-all-trades at several points in the movie, showing skill in areas such as science, history, and disguise (though his disguise inevitably fails), not to mention being physically capable (this is emphasized in the film's climax in the Big Ben Tower).
Despite this, Basil can also be prickly and self-centered, ignoring Olivia and Dawson at the beginning of the movie, and only offering to help them once he realizes Olivia's case might be the solution to finally capturing Ratigan. Though Basil is always calm and collected during his hunt, he can also be very emotional and operatic, becoming very moody if things do not go his way or if the situation looks dire, such as in the beginning of the movie and when he is nearly defeated by Ratigan. Despite his indifferent attitude towards Dawson and Olivia, he can be quite affectionate, as shown when he tries to cheer up the former after losing Fidget in the Toy Store, and also when he admits that Olivia is 'not such a bad girl'.
In the film, when Dr. Dawson meets a lost and frightened Olivia Flaversham, desperately searching for Basil to help her locate her kidnapped father, he helps her find Baker Street. Before long, they are involved with Basil in a maze of mystery and intrigue, trying to see if he can pinpoint the location of his arch-nemesis, the nefarious Professor Ratigan. Basil has fought Ratigan before, but he has never been able to catch him, and knows he is at large, ready to commit another diabolical crime. Basil initially ignores Olivia as he is trying find a match between two different bullets that might help him find Ratigan, but he (again) fails and walks over to his chair, dejected. As he sits down and starts playing his violin sadly, Basil finally takes actual notice of Olivia when she mentions that her father was taken by a peg-leg bat, who Basil immediately recognizes as Fidget, Ratigan's main henchman. Olivia tells Basil her story, and while Basil is trying to figure out what Ratigan wants with Olivia's father, Fidget turns up at the house, but Basil is alerted to his presence by Olivia, prompting him and Dawson to rush out to find the bat.
Unfortunately, the bat has escaped but left his footprints on the ground. Basil concocts a plan to find him, but when Olivia almost breaks his violin he doesn't want her to come (because he knows detective work was too dangerous for children); however, he ends up breaking it upon sitting on it. He gets enraged at this, but he somehow finally does allow her to come anyway. After seeing Sherlock Holmes leave the house, Basil calls for Toby the dog. The dog comes and Basil is slightly annoyed that the dog seems to listen to Olivia more than him. He is able to convince the dog to help him find Fidget, and the dog, carrying Basil, Dawson, and Olivia, tracks Fidget's scent as he runs across the town. Eventually, they arrive at a toy store, and the three mice then sneak into the store.
Basil finds that the royal guard suits have been stolen, and searches the store for Fidget. He is disturbed by Olivia activating the toys and orders Dawson to watch her. As he investigates, Dawson finds a list of things Fidget has to steal, and tries to tell Basil about it, but he is preoccupied with his task to listen. During the distraction, Olivia gets lured away by Fidget and gets kidnapped. Basil and Dawson go after Fidget, but Dawson gets pinned to a board by a dart, leaving Basil to chase Fidget alone. Fidget cannot escape out of the window because of Toby's presence, so he climbs up a pile of blocks to get to the ceiling window. Basil climbs up the pile in pursuit of Fidget, but the pile collapses, and Basil falls back down while Fidget escapes with a now-captive Olivia.
Basil blames Dawson for letting Fidget escape, but soon forgives him (after realising he has hurt Dawson's feelings) and tries to think of another plan to locate Olivia. However, upon seeing that Dawson has Fidget's list, Basil has an epiphany as the list contains some necessary clues. After he and Dawson return to Baker Street, Basil analyzes the paper and deduces that Fidget was at the bar and, from doing some chemistry, the paper was highly saturated with distillation of sodium chloride. The results of analysis lead Basil to a pub where the sewer meets the waterfront, and he and Dawson immediately go there disguised as sailors. When he mentions the name Ratigan to the barmaid, he and Dawson's drinks are drugged. Basil is able to find out, but Dawson, who has drank the drug-induced drink, ends up causing a bar fight. Amid the chaos, Basil finds Fidget and gets Dawson to follow him to a trapdoor he has found.
Seeing Fidget go into a drainpipe, Basil and Dawson climb into the pipe and follow him to Ratigan's lair. However, upon arrival, the two mice are captured by Ratigan and his thugs. As the thugs pin Basil and Dawson to a mousetrap, Ratigan explains the functionality of a very complicated death trap he has constructed to kill the two before stating that he has an important engagement at Buckingham Palace. After Ratigan leaves Basil and Dawson to die, Basil has a nervous breakdown from being finally outwitted by his arch-nemesis but Dawson talks some sense into him, suggesting that they set off the trap now. At these words, Basil has another epiphany, and as the metal ball released from when the record stops playing starts rolling down the ramp towards them, Basil instructs Dawson to release the triggering mechanism at his signal. They do, and the metal ball gets caught in the mouse-trap's spring bar, leading to a chain reaction that results in Basil, Dawson, and Olivia (who was trapped in a bottle earlier by Fidget) breaking free from their captivity.
After leaving Ratigan's lair, the three mice ride Toby to Buckingham Palace, arriving just in time save the Queen from being eaten by Ratigan's pet cat Felicia (whom Toby chases after). As Olivia reunites with her father, Basil and Dawson expose Ratigan and subdue his henchmen, but Ratigan captures Olivia again and retreats. Basil, Dawson, and Olivia's father rush outside the palace and improvise a flying apparatus out of a flag of Great Britain, some balloons and a matchbox for the three mice to give pursuit. During the chase, Fidget is thrown into the Thames River, and Basil jumps into Ratigan's airship, which then crashes into Big Ben.
As Basil awakens to find himself in the innards of the clock tower, Ratigans stalks him from behind, preparing to ambush him, but Olivia distracts him long enough for Basil to tangle Ratigan's cape into the clock's gears. Noticing Olivia in danger of getting crushed, Basil rushes to her rescue and manages to return her to her father before getting tackled by a now-highly savage Ratigan. The two fall onto the clock's hour hand, and Ratigan viciously swipes Basil with his razor-sharp claws, eventually knocking Basil off the hand. However, Basil manages to grab ahold of a propeller of Ratigan's airship that was destroyed when it crashed into the clock. Just then, the clock strikes ten, and Ratigan tumbles off the hand, grabbing Basil by his jacket. The two arch enemies fall from the tower, but Basil saves himself by using the propeller he had held onto to propel himself back up.
Having saved the Queen and her kingdom, Basil is honored by the Queen, along with Dawson, as shown in a photo on a newspaper that he has kept. After saying goodbye to Olivia and her father who leave to catch the train, Basil and Dawson are visited by a distressed lady mouse who has earlier lost her ring. From this moment on, Dawson becomes Basil's assistant, and they solve many more crimes together.
Basil made a few cameo appearances in House of Mouse. His most notable cameo is him keeping Dawson away from a mousetrap that Ratigan has set for them in the episode "Donald's Lamp Trade". In another episode, Basil and Dawson were some of the mice to take flight on Dumbo's Airline.
Basil was a supporting character in the Disney Adventures comic, where Olivia and the newly reformed Fidget took the starring roles.
Basil and Ratigan made meet-and-greet appearances throughout the theme parks in promotion for the film. Tradgically, They have been retired since 2004.
- It was actually originally intended for iconic British actor John Cleese to do the voice of Basil but turned down the role since he was cast in the Non-Disney Live Action film, A Fish Called Wanda, released in 1988.
- The way Basil stands up straight with his fist clenched to Ratigan is the same position Roger stood up to Cruella De Vil in 101 Dalmatians.
- Basil is named after Basil Rathbone, the actor who portrayed Sherlock Holmes numerous times.
- Basil's mother made a cameo appearance on a Dutch illustration called 'Uit Het Plakboek van Basil Holmuis' ('From the scrapbook of Basil of Baker Street').
- Merchandise depicts him with a green coat, which he is never seen wearing in the actual movie.