The novel begins with two of our main characters walking on a famous beach. One is Charles Smithson, a wealthy man from London with an interest in paleontology. The other is Ernestina Freeman, wealthy as well, and a well-regarded socialite. The two are engaged to be married, and spending time together in the city of Lyme Regis. At the beach, they encounter Sarah, or “the French Lieutenant’s Woman” as she is more commonly known. Her reputation was lost when she had an affair with a Frenchman who came to the country shipwrecked. At the time of the affair he was staying in the house where Sarah tutored children. He had promised her he would marry her and then disappeared. Now whenever she gets the time, she sits on the beach waiting for him to come back. Sarah has now been living with a rich old lady called Mrs. Poulteney as her companion for a year.
The sadness in her eyes disturbs Charles. Charles runs into Sarah twice more and each time she asks to be left alone. The first is in the Undercliff, where Charles had gone to look for fossils. Sarah goes there often despite Mrs. Poulteney asking her not to because of its reputation as a place where immoral things are done.
The couple and Ernestina’s Aunt Tranter go to visit Mrs. Poulteney. Sarah is there but she doesn’t speak. The four discuss the implications of Mary, Mrs. Tranter’s maid, and Sam, Charles’ manservant, falling for each other and how inappropriate it is. Charles disagrees with the rest and thinks it alright, which leads to a small fight between him and Ernestina that is quickly forgotten. Charles begins to wonder if he is making the right choice marrying Ernestina, or if he’s just doing it because it’s what’s expected of him.
Meanwhile, Charles and Sarah keep running into each other at the Undercliff when he’s out looking for fossils. The Tragedy, as she’s known to some, actually fascinates Charles, but he also feels very sorry for her. Once when he crosses paths with her, he offers to help her find a job outside the city where no one knows her, but she doesn’t take him up on it. The next time, she tells him she wants to relay the story of what happened with the French sailor, to Charles’ surprise. She says she needs to tell someone who will understand instead of judging her, so Charles listens. After the story, Charles cannot stop picturing Sarah giving herself to the sailor, and he realizes they are attracted to each other. Charles tells Sarah they shouldn’t meet alone anymore, and later asks her to leave the city altogether, saying he will help pay for it. Sarah agrees.
One day Charles’ Uncle Robert calls for him. When he goes to visit him, he finds out that he is going to get married soon to a younger woman, which means that if they have a son, the family’s money goes to him instead of Charles. Ernestina is infuriated by the news, and Charles is devastated that he will now have to rely on her money after they are married.
Charles comes back to Lyme to the news that Mrs. Poulteney had let go of Sarah, who disappeared soon after. He finds a note from her begging to see him one last time. He goes to Dr. Grogan to ask for advice on what to do. Dr. Grogan says he will go himself, and take her to an asylum where she can recover from her melancholia. Charles goes back home upset about what Sarah’s done but comes to the conclusion that he and the doctor have judged her wrongly. To make things right, he decides to go find her.
He finds Sarah sleeping inside a barn. Not being able to help themselves, they share a kiss, but Charles pushes Sarah away and leaves in a hurry. On his way, he runs into Sam and Mary and asks them to not tell anyone about it. Charles comes back to Sarah to give her some money so she can leave the city for good. They say their goodbyes.
Charles tells Ernestina he has to go talk to her father about the new developments regarding his finances. Sarah moves to Exeter. Mr. Freeman tells Charles he can start working in the family business, even though gentlemen don’t usually work. Dreading what the future holds for him, Charles goes to his club to get really drunk. He ends up going to a brothel with old friends from Cambridge but leaves without doing anything. When he’s going back home he notices a prostitute that looks somewhat like Sarah and decides to hire her. Charles plans to go through with the act but when he finds out her name is actually Sarah, he pukes on her bed. She takes care of him. The next morning, he gets a note from Sarah with the address of where she’s staying in Exeter.
Charles is on the train to Exeter from London thinking about what he’s going to do. He imagines the path he’s set on; a boring marriage, kids and working for his father-in-law. Instead of taking the connecting train to Lyme, he decides to stay for a night in Exeter. He goes to see Sarah and they end up having sex. When he’s getting dressed, Charles notices a spot of blood on his shirt, meaning that he had just taken Sarah’s virginity. The story about the French lieutenant was all a lie. Charles is confused why Sarah would ever have made it up, but she refuses to answer his questions. All she says is that she loves him but doesn’t ask him to leave his fiancée for her. Charles leaves angry.
While trying to walk off his anger, Charles ends up at a church. He prays even though he is an atheist. After giving it proper thought, he realizes he can’t make his life choices based on what others will think of them. He pictures what his life with Sarah will be like instead. He goes back to his hotel and writes a letter for her, and asks Sam to deliver it. Sam doesn’t do it because it might hurt his chances of getting to marry Mary and open his own store if Charles doesn’t lend him money from the Freemans.
Charles tells Ernestina everything. She threatens him and says her father will ruin his reputation. She collapses and Charles gets Dr. Grogan to help. The doctor is extremely upset when he hears what Charles has done.
Charles tries to contact Sarah but finds out she left her hotel in Exeter without providing an address of where she’s going. He signs an admission of guilt for Mr. Freeman and tries to go find Sarah. After failing, he travels Europe for nearly 2 years. Despite this being what he’s wanted for some time, he feels empty without Sarah by his side. He ends up visiting America, which is when he receives a letter from his lawyer saying he’s found Sarah.
Two endings are described in the novel. The first ending has Charles visit the address that was given to his lawyer anonymously, and it’s quite a nice house. It is the home of an artist Dante Gabriel Rossetti. Inside, he finds Sarah wearing modern clothes. She tells him she is the painter’s assistant as well as a model for his work. Their relationship is purely professional. Once Charles understands this, he begs Sarah to leave with him and for them to get married. Sarah refuses to marry him; she finally feels like she belongs and she is happy with how her life has turned out. She doesn’t need saving. Charles doesn’t buy it, he thinks she’s just hiding her pain. He then accuses her of calling him to torture him. Sarah doesn’t lose her cool attitude, she says there is a reason she called him. He has to meet someone. Charles is reluctant but eventually agrees to it, when a little girl is brought out. She is his daughter, conceived when he took Sarah’s virginity at the hotel. Although it isn’t explicitly said, it looks like the three will be together as a family from now.
The other ending in the novel is brought about by the narrator being inserted into the novel as a character and this is a much sadder one. The narrator is outside the painter’s house and turns back time 15 minutes by turning the hands of his pocket watch, before leaving in a carriage. This is right when Charles was accusing Sarah of only bringing him back to hurt him. He gets up, and although Sarah tries to stop him from leaving, he storms out of the Rossetti house. Sarah and Charles do not end up being together, and Charles never meets or knows of his daughter. Charles realizes life is only meant to be endured, there is no real happiness or solution to the emptiness. There is no fix for the problems of life.