This is the fourth book in Margery Sharp's fantasy series "for all ages". As always, the central characters are the two mice, Miss Bianca and Bernard, and various other members of the Mouse Prisoners' Aid Society; and, as usual, the plot revolves about Miss Bianca's kindhearted determination to rescue some poor human from undeserved durance vile in an absolutely escape-proof prison -- this time, it's a little boy, Teddy-Age-Eight, enslaved in the salt mines. As before, Miss Bianca is sophisticated and charming, and Bernard is devoted and persevering; the plot and dialog skip wittily along, and a happy ending is soon reached for all.
It's not that I didn't enjoy the book, but I was disappointed to discover that it didn't say anything new. Miss Sharp has written the same book for the fourth time now, and the novelty of the basic plot has worn off. The series has not deteriorated, exactly, but it is noticeably beginning to stagnate.