Beneath the house of the esteemed detective, Sherlock Holmes, lives his mouse counterpart the equally infamous, Basil of Baker St. In another part of London, a little girl, Olivia, is celebrating her birthday with her father. When Olivia's father is kidnapped, the young mouse goes in search of the one and only mouse detective. Along the way, she meets up with a Dr. Dawson, recently returned from services overseas. He helps the girl find Baker St. and together they ask Basil for his help in tracking down Olivia's father. Basil deduces that her father was kidnapped by none other than his arch-rival, Ratigan, and agrees to take the case. What Basil doesn't know is that Olivia's father is only one piece is Ratigan's grand plan to take over all of Mousedom.