Martin Urban is a quiet bachelor with a comfortable life, free of worry and distractions. When he unexpectedly comes into a small fortune, he decides to use his newfound wealth to help out those in need. Finn also leads a quiet life, and comes into a little money of his own.

Normally, their paths would never have crossed. But Martin's ideas about who should benefit from his charitable impulses yield some unexpected results, and soon the good intentions of the one become fatally entangled with the mercenary nature of the other. In the Lake of Darkness, Ruth Rendell takes the old adage that no good deed goes unpunished to a startling, haunting conclusion.