Although set in three movements, the material expounded at the outset is developed throughout the sonata. The work is linked, extra-musically, to the earlier Vyrnwy Sonata for harp, in that it takes as its inspiration the atmosphere and folk legends of Lake Vyrnwy in Powys.
The opening movement presents the material of the sonata and can be seen as representing the forces of Nature so evident at the lake, in particular, the bird-life and the spectacular and rapid climatic changes.
The second movement is, as in the Harp Sonata, a Nocturne, brooding on the submerged town, drowned beneath the lake.
The lively scherzo of the last movement depicts the “Fire Spirit” who inhabits the valley and dances around causing mischief.