Lida Stearl prides herself on always knowing the most appropriate thing to do in any given situation—confidence that has served her well in building her career as an orthodontist, maintaining a happy marriage, and raising her young niece in the wake of a violent tragedy. But now she's a widowed, retired empty-nester and the small perfections of an orderly life are not quite enough to stop her from feeling adrift.
Then a well-intentioned birthday gift leads to the discovery that Clarence Lusk, on death row for the murder of Lida's sister, is seeking penpals from the outside. And so, for the first time in her life, Lida crosses a line: she begins to write to him, pretending to be naive, twenty-three, and just the slightest bit flirtatious. As letters pass steadily between Lida and Clarence, her preoccupation with his crime and its echoes intensifies, and she finds that crossing one line makes the ones that follow all the more tempting to cross.