Queen Mousetoria is first seen in a photo of a newspaper article that is announcing her Diamond Jubilee. Ratigan shows this photo to his henchmen while debriefing them on his scheme to seize control of Mousedom and presently burns it with his cigarette.
She physically appears near the end of the film, excitedly primping herself in preparation for her jubilee. When Ratigan's thugs, disguised as her guards, show up and present her with the robot facsimile of herself that Hiram Flaversham built, Queen Mousetoria is delighted to be receiving a gift but becomes confused when she sees the note attached to it says that her reign will come to an end.
When Ratigan enters the room, Queen Mousetoria, still believing his disguised henchmen to be her actual guards orders them to seize him. However, their boss gives them the same order and they seize her instead, prompting her to shout "how dare you!"
While Ratigan's coronation is taking place, she is carried off by Fidget in order to be fed to Felicia, who is eagerly waiting outside one of the palace windows. She very nearly becomes the cat's dinner, but at the last minute, Basil runs up from behind and grabs her out of Fidget's grasp. A little later, Queen Mousetoria is seen tying Fidget up and trying to restrain Ratigan.
She makes her last appearance in a photo of another newspaper (kept by Basil) in which she honored Basil and Dawson in gratitude for saving her life and her kingdom.
Her name is a portmanteau of "mouse" and the real name "Victoria". As such, she is the mouse equivalent to the monarch of Britain at the time that the Sherlock Holmes and Basil of Baker Street books take place, Queen Victoria.