So Long a Letter recounts two different paths taken by Senegalese women whose choices are extremely limited. The novel discusses the stories of lifelong friends Ramatoulaye (who is writing the letter) and Aissatou and the ways that their relationships with men are both similar and different.
Ramatoulaye ends up marrying Moudou Fall, who eventually becomes enamoured with their daughter's young friend Binetou. Moudou takes Binetou as his second wife, and Ramatoulaye is forced to choose divorce or living as a co-wife. Ramatoulaye decides to stay with Moudou, and she explains in her letter how she finds a way of surviving, working, and raising her children in this difficult situation.
Aissatou, on the other hand, marries Mawdo. Mawdo eventually gives in to pressure from his family to take a second wife. Contrasting with Ramatoulaye's decision, Aissatou divorces her husband, refusing to take part in this oppressive patriarchal tradition of co-wives.
Binetou, Moudou's second wife, also has a interesting and important story in this book. We learn that Binetou comes from an extremely poor family, and that, because her family needs the financial support, she also has little choice in marrying Moudou.
The novel ends with Ramatoulaye reflecting on modernity and the changing world. She reflects on how her children's paths are even more complicated, and perhaps even more hopeful, than hers and Aissatou's. Ultimately, So Long a Letter is about women living in a world transitioning between old religious cultures and the new cultures of globalization and feminism.