In Victorian England, a young biologist named Charles Smithson finds himself in the midst of a personal and social dilemma. About to become engaged to the sweet and traditional Ernestina Freeman, his life takes a radical turn when he meets Sarah Woodruff, an enigmatic woman who lives outside the bounds of social norms. Sarah, nicknamed "the French lieutenant's woman," is seen by many as insane due to her rejection of the conventional expectations of the time. Intrigued by her story and mysterious aura, Charles feels drawn to her, seeing in her something that challenges his own convictions and desires.
As his relationship with Ernestina progresses, Charles increasingly immerses himself in Sarah's world, which reveals to him a more intense and authentic perspective on life. Through secret meetings and deep conversations, they both explore their personal aspirations and frustrations, leading them to question the norms of society and the roles imposed upon them. However, Charles's internal conflict intensifies as he must decide between following the safe path that offers a conventional life with Ernestina or pursuing the passion and freedom that Sarah represents.
The story navigates a dance between the classic and the modern, showcasing the struggle of a man caught between the expectations of his environment and the burning desire to be true to himself, culminating in an open ending that leaves the audience reflecting on love and life choices.