The title of the novel The Last Girls by Lee Smith refers to the custom of the time during which the novel is set (i.e., the 1960s) to call women of adult age "girls" and not "women," thus implicitly denying women their maturity in age or in experience.

It revolves around the characters of Harriet Holding, a teacher who has remained unmarried for reasons that she herself doesn't fully understand; Courtney Gray, a Southerner who is struggling to escape her conservative upbringing and picturesque lifestyle; Catherine Wilson, a sculptor who appears to be content in her third marriage but feels a sense of dissatisfaction with no immediate material cause; Anna Todd, a world-famous novelist who is using romance fiction as a channel for her own suffering and escape; and Margaret "Baby" Ballou, a rebellious, free-spirited creature. They are all college friends.

The story opens in the mid-1960s during a trip to Mississippi the girls had taken on a homemade raft. The raft trip is inspired by Mark Twain’s famous Adventures of Huckleberry Finn, a book the group had taken interest in after a handsome professor taught the text. Their adventure is closely documented by the press, who refer to them as "girls" and not "women" in spite of their ages. As the Kentucky paper’s headline reads, “It’s Girls A-Go-Go Down the Mississippi.” This is a sign of the times. The narrative then shifts to the present day, when all the girls once again gather to go on another trip, with the noted absence of Baby. We learn that Baby has died, and it is heavily implied to have been a suicide. Prior to her death, Baby had told her husband that she wanted to recreate that first raft trip with her friends. As such, Baby's husband requests the girls to scatter her ashes in that same river.

This time around, the women ride on the Belle of Natchez, a cruise ship that will take them to New Orleans. On the journey, we learn that Baby was a promiscuous Southern belle, contrasted with her shy roommate, Harriet. Harriet had introduced Baby to a friend, Jefferson Carr, in college, whom Harriet had feelings for. However, Jefferson had instead been drawn to Baby. Because of her capricious nature, Baby did not commit to Jeff as he had done to her. Harriet attempted to intervene and reunite the couple, yet ended up sleeping with Jeff instead. Heartbroken over the whole ordeal, Jeff joined the Army and was killed in a helicopter crash. This caused a strain in Harriet's friendship with Baby, and has subtly weighed down upon Harriet’s ability to marry ever since.

It becomes clear that Baby was more dynamic than initially perceived—Harriet also brings along some of Baby’s poetry from their college days. Baby expresses a great deal of grief surrounding her late mother, her younger brother, and her picture-perfect life that feels like a facade.

We also learn the stories of the other characters: such as Anna's literary pursuits, Courtney's love affairs, and Catherine's troubles with her immature husband. The story provides an insight into the ways youth, romance, and friendship shape us over a lifetime.