What is the main conflict in The French Lieutenant's Woman by John Fowles?

Charles learns of the local outcast, Sarah Woodruff, known also as “The French Lieutenant’s Wife” and “Tragedy”. Sarah is employed by Mrs. Poulteney, an elderly widow who has taken her on as a secretary. Mrs. Poulteney is obsessed with her salvation, and with performing acts of good works to ensure it.

Charles and Sarah meet alone accidentally a number of times, and Sarah ultimately reveals her history to Charles. She had hoped to marry a French lieutenant, injured in a shipwreck, who was recovering at the home of the family to whom she was a governess. She gave herself to him sexually, only for him to leave the next day. He later married someone else, and never returned.