In Victorian London, the film follows Basil of Baker Street, a clever and daring mouse detective who emulates Sherlock Holmes' brilliant detective skills. Basil's world is turned upside down when he is approached by Dr. Dawson, a newly arrived mouse to London, and Olivia Flaversham, a young Scottish mouse seeking help to find her father, Hiram, a skilled toymaker who has been kidnapped. The disappearance of her father is linked to the nefarious Professor Ratigan, a ruthless and cunning criminal mastermind, who aims to overthrow the mouse kingdom’s monarch. Basil, recognizing the gravity of the situation and the cunning of his adversary, accepts the challenge, embarking on a dangerous adventure.

As Basil, Dawson, and Olivia delve deeper into the mystery, they navigate through various obstacles and pitfalls designed by Ratigan to thwart their investigation. Utilizing a blend of intellect, bravery, and innovative gadgets, they explore the shadowy streets of London, gather clues, and slowly unravel the connection between Hiram's abduction and Ratigan's sinister plan. Their journey is fraught with danger, involving thrilling chases, devious traps, and confrontations with Ratigan's henchmen. Through their adventures, they also forge a deep friendship, learning the value of teamwork and resourcefulness in the face of adversity.