Bianca and Bernard, agents for The Prisoners' Aid Society of Mice, rescue prisoners and outwit villains in this enchanting story, made world-famous by the Walt Disney film. The Prisoners' Aid Society of Mice discusses the proposed rescue of a Norwegian poet from the terrible Black Castle. Miss Bianca, the pet white mouse belonging to the Ambassador's son, is sent to Norway on a mission to recruit the bravest Norwegian mouse she can find.
She finds Nils, and brings him back triumphantly. Then she, Nils, and Bernard, a pantry mouse who falls in love with her, set off for the Black Castle. They set up home in a mousehole in the Chief Jailer's room, and narrowly avoid the jaws of Mamelouk the cruel Persian cat.
Eventually they trick the cat and the jailer, and get into the prisoner's cell.
A dramatic rescue via an underground river, and they are all free - and the Nils and Miss Bianca medal for bravery is struck in the mice's honour!