The mystery began for the Hadley children when they went to the airport for the arrival of the famous violist, Nicolas Matthias. They met Horace Elik who had a strange friendship with the violist. Then the violist became ill and spent the night at the Hadley home. That wasn't so mysterious until Dr. Lindsay said that Mr. Matthias wasn't ill at all. Events piled up which plunged the children into a new adventure. There was the sonata that Horace Elik was anxious to get his hands on, and the music group which held strange meetings. And there was Toby, the young crippled boy who wanted to make violas. The story is full of suspense as it draws to an astonishing conclusion.