When the chairwoman mouse informs delegates about the case of a Norwegian poet, imprisoned (presumably wrongfully) in the horrible Black Castle, she suggests changing the traditional rules of the organization to try to rescue him.

Bernard, a young mouse who lives in the embassy pantry and who has won a medal for "bravery in the face of cats", is volunteered by his friends and reluctantly agrees to talk to Miss Bianca.

Upon arrival in Oslo, Miss Bianca escapes the little boy's room and enters an underground wine cellar full of sailor mice at a bachelor party.

He takes Miss Bianca back to the Prisoner's Aid Society, the first half of the journey stowing away on a cargo ship and the rest of the way by speedboat (a toy of the Ambassador's son).

However, expeditions by Bernard and Nils show the mission is nearly hopeless—all the doors in the castle are heavy iron, there are very few places to hide, and they are constantly in danger of Mamelouk, the Head Jailer's gigantic black half-Persian cat.

At first, Miss Bianca serves as de Facto chairwoman, but soon begins to take a more active part in the mission when she encounters Mamelouk and discovers her charm and wit both infuriate and fascinate him.

Bernard and Nils, now able to explore more freely with Mamelouk distracted, discover that the river that runs below the castle, swollen by a sudden storm, has washed away rocks blocking up an old water gate.

The three mice are welcomed back to headquarters amidst a huge celebration, and several artifacts of their time in the Black Castle are framed in the meeting hall for posterity.

Reviewer Meghan Cox Gurdon of The Wall Street Journal noted that the book "is much funnier and more interestingly textured than the high-fructose movie version.