Voices in the Garden

First published 1981, this is Dirk Bogarde's second novel.

The fabulous but wavering old Lady “Cuckoo” Peverill, lives with her husband, Napoleon-mad military historian, Archie. Dissatisfied and overcome by sheer boredom, she ventures down to the lake at the edge of their estate, pockets filled with stones, she begins to walk into the water. Before she is too deeply submerged, she is pulled away by sparsely-clothed drifter, Marcus Pollock.

Feigning that he merely saved her from an 'accident'...