"The Fascinating Problem Of Uncle Meleager's Will" is a short story from Dorothy Sayers' book, Lord Peter Views the Body.
This is apparently the first Lord Peter story; it was originally published in Pearson's Magazine (July 1925).
When old Meleager Finch dies, he leaves two wills: the first grants his fortune to the conservative Primrose League, while the second leaves everything to his niece, Hannah, a young, serious socialist. He writes a letter to Hannah explaining that this second will has been hidden somewhere, and that her only chance of finding it is "by the exercise of a sustained frivolity."
Lord Peter, his sister Mary, and his valet Bunter travel with Hannah to help her find the missing will. They discover that Meleager was a crossword puzzle fiend, and find a trove of puzzles he had written, including one with no accompanying grid. When Peter falls into the impluvium, he discovers that the missing grid is the tiled floor of the fountain itself, and solving it reveals the location of the missing will.
The story is notable for containing an entire crossword puzzle in which all the clues are rhyming couplets (or quatrains).
