The narrator begins this story by saying it is the saddest story he knows.
Part One
We learn of two couples who are all friends: John Dowell, the narrator, and his wife Florence, and their couple friends, Edward and Leonora. They are from Connecticut, but originally their roots go back to the United Kingdom. The women in both couples suffer from heart issues, so they often meet at a spa that is soothing for the women. When they visit the town where Martin Luther's protest is kept. Suddenly Leonora realizes that Florence and her husband Edward are paying each other romantic attention. After their nine year long friendship, Leonora is confident they will ruin it all to sleep together.
Edward is prone to affairs, having had many of the (we learn of a railway car incident and various affairs with the mistress of the Duke of Monte Carlo, the wife of a military officer—in fact the entire reason the two families are connected is because Florence saw Leonora attacking a woman suspected of sleeping with Edward.
Leonora invites that woman (the mistress whom she attacked) as a decoy for Edwards wanton appetites, so he will betray Florence, but the lady, Mrs. Maidan, overhears a telling conversation, and she literally dies from the horror and shame.
Part Two
In a flashback, Dowell tells about his wife. She has always wanted to be the lady of Fordingbridge for personal reasons pertaining to her family history (where Edward is from, it turns out). Dowell tells that even though they were married against her family's will, an indication that she loved him, she actually has never had sex with Dowell. She did once cheat on him with a pedestrian artist named Jimmy.
We learn that Florence and Edward have already been having an affair through the entire duration of the 9 year friendship. She plans to divorce Dowell until one day, she sees Edward with yet another young woman, so she runs to tell Dowell what she saw, but when she arrives, she learns that Dowell has learned from an old friend about her affair with Jimmy. Later that night, she poisons herself and dies.
Part Three
Dowell considers marrying Nancy, but Edward loves her too (the reader knows because Nancy was the girl Florence saw him on a date with). Nancy is Edward's niece, so she doesn't realize his intentions are romantic. They all four travel to Branshaw Manor, where Leonora has a nervous breakdown, unable to cope with the latest affair. We learn of all the affairs in more detail.
Part Four
When Leonora tells Nancy all the secrets she keeps about Edward's faithlessness, Nancy is heartbroken. The two work together to punish Edward emotionally for his constant failures and indiscretions. The subtle abuse drives him crazy, and when Nancy goes on a trip with her father to India, he is desolate. Her letter mentions no romantic feelings toward him; quite the contrary, she is very happy without him. He kills himself with a pocket knife.
Leonora remarries in the epilogue, but Nancy loses her mind. Dowell buys Branshaw Manor and takes care of Nancy.
