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 to find Baker Street. Before long, they were involved with Basil in a maze of mystery and intrigue, trying to see if he can pinpoint the location of his arch-enemy, the nefarious Professor Ratigan. Basil has fought Ratigan before, but he has never been able to catch him, and knows he was out there, ready to commit another diabolical crime. Basil ignores Olivia, as he was trying find a match between two different bullets that might help him find Ratigan, but he fails and sits in his chair and starts playing his violin sadly. He 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 he's trying to figure out what Ratigan wants with Olivia's father, Fidget turns up at the house, but Basil was alerted to his presence by Olivia, and he and Dawson come out to find him.
Unfortunately, the bat has escaped but left his footprints on the ground. Basil comes up with 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), but he ends up sitting on it and breaking it. He gets enraged at this, but he somehow finally does allow her to come anyway. Basil sees Sherlock Holmes leave the house and calls for Toby the dog. The dog comes and Basil was slightly annoyed that the dog seems to listen to Olivia more than him. He was able to convince the dog to help him find Fidget, and the dog captures Fidget's scent and runs across town to a toy store, where Basil, Dawson, and Olivia then sneak into.
Basil finds that the royal guard suits have been stolen, and searches the store in search for Fidget. He was disturbed by Olivia activating the toys and orders Dawson to watch her. As he investigates, Dawson finds a note of the list of things Fidget has to steal, and tries to tell Basil about it, but he won't listen. During the distraction, Olivia gets lured away by Fidget and gets kidnapped. Basil and Dawson chase Fidget but Dawson was pinned to a board by a dart, leaving Basil to chase Fidget alone. Fidget cannot escape out of the window because Toby was there, so he climbs up a pile of blocks to get to the ceiling window. Basil nearly gets him, but the bricks tumble down and Basil was almost crushed underneath, allowing Fidget to escape.
Basil blames Dawson for letting Fidget escape, but soon forgives him (after realising he had hurt Dawson's feelings) and tries to think of another plan to now locate Olivia. After seeing the list, he figures out that Fidget was at the bar and comes there disguised as a sailor with Dawson. When he mentions the name Ratigan to the barmaid, he and Dawson's drinks were drugged. Basil manages to find out the drinks were drugged, but Dawson was drugged and ends up causing a bar fight and falls down on top of the root beer mug and lay his stomach on top of the root beer mug. During the fight, Basil and Dawson see Fidget and follow him to Ratigan's lair, but it turns out to be a trap and he and Dawson we were captured and Ratigan reveals he plans to use a robotic version of Queen Moustoria and capture the real queen and the robot will name him king.
Ratigan leaves Basil and Dawson to die as he constructed a very complicated death trap to kill him, but Basil was able to use that trap to free himself, Dawson, and Olivia. They ride Toby to Mousedom where Ratigan has been named king, and they save the queen, subdue the henchmen and expose Ratigan, who kidnaps Olivia again and retreats. Basil flies after him, and Fidget was thrown into the water and Basil and Ratigan crash in Big Ben. They awaken and fight each other, and Basil rescues Olivia and returns her to her father and continues his fight with Ratigan. The fight ends with both Basil and Ratigan falling from Big Ben, but Basil was able to survive by using a piece of Ratigan's balloon to fly back up while Ratigan dies. Basil was rewarded by the Queen and Olivia was returned to her father, and 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.