It’s 1860, and Roger Button’s wife has just given birth to their first child. The Buttons are respected members of Baltimore society, so it’s surprising that their family doctor, Doctor Keene, brushes off Mr. Button when the men run into each other outside the hospital. He storms away and says that his reputation as a doctor has nearly been ruined. Inside the hospital, Mr. Button encounters the same hostility from the nurses, and he soon sees why: his newborn son looks and acts like a 70-year-old man.
The nurse orders Mr. Button to take his son home as soon as possible, since the baby’s unusual condition could threaten the hospital’s reputation. Mortified by the idea of being seen in public with Benjamin, Mr. Button rushes out to a clothing store to buy baby clothes for his son, though Benjamin looks ridiculous in them. The Buttons eventually decide to name their son Benjamin.
Mr. Button dyes Benjamin’s hair, trims his beard, and tries to force Benjamin to live like an average child. Still, all Benjamin wants to do is read an encyclopedia, smoke cigars, and chat with his grandfather. He does make a point of breaking something in the house each day, knowing that it pleases Mr. Button to see his son acting like a little boy.
By the time Benjamin turns 18, he doesn’t look quite so old, though he still needs to dye his hair. He runs out of hair dye shortly after arriving at Yale University, where he has been admitted as a freshman. The registrar thinks he’s a strange older man posing as a college freshman, so he runs him out of his office. As Benjamin leaves, students follow him and shout insults, telling him to go try this crazy stunt at Harvard. Boarding a train for Baltimore, he decides that he will go to Harvard someday.
Back at home, Benjamin joins his father’s hardware business, becomes wildly successful over the next few years. When Benjamin is 20, he looks strikingly like his father. As wealthy business partners, they attend prestigious dances together. At one of these high-society events, Benjamin meets Hildegarde Moncrief, the daughter of General Moncrief, one of the most powerful men in town. As Benjamin and Hildegarde dance, she tells him how much she likes older, more mature men—men like Benjamin. According to her, 50 is the ideal age. Benjamin doesn’t tell her about his condition, instead enjoying her affection. Not long after this, they announce their engagement.
With news of Benjamin and Hildegarde’s marriage, Baltimore erupts in gossip about Benjamin’s condition. This time, though, people spread false rumors about him. General Moncrief hates the idea of Hildegarde marrying Benjamin, but he eventually gets used to it, largely because Benjamin is a successful businessman. Within 15 years, Benjamin is not only the head of the company, but also has the body and mind of a handsome young man. All the gossip about him goes away.
The only problem, though, is that Benjamin is no longer attracted to Hildegarde, who is now 35. Together, she and Benjamin have a son named Roscoe, but Benjamin dislikes their boring family life. Seeking a thrill, he joins the army and goes off to fight in the Spanish-American War, where he serves as a high-ranking official. After sustaining a small injury, he returns to Baltimore as a decorated and celebrated veteran.
Home from the war, Benjamin clashes with Hildegarde over his reverse aging and his growing interest in pretty young women. Meanwhile, Roscoe grows up and starts to look the same age as his father, and Benjamin hands the business over to him. Then, when Benjamin looks 20 years old, he attends Harvard University, where he becomes the star of the football team. But his athletic prowess doesn’t last long, since he becomes weaker by the year. By the time he’s a senior, he can’t even make the team.
Still, Benjamin graduates from Harvard. He then returns to Baltimore and moves in with Roscoe, since Hildegarde has moved to Italy. Roscoe is ashamed that his father looks like a teenager, so he tells Benjamin to start referring to him as “Uncle” when other people are around.
Benjamin continues to grow younger, and he goes to kindergarten at the same time as Roscoe’s young son. But when Roscoe’s son moves on to first grade, Benjamin stays behind. Before long, Benjamin can’t even keep up with the other kindergarteners, so he stays home with a caretaker he calls Nana. Soon, Benjamin only registers simple sensory details; memories of his adult life never occur to him. There’s only Nana’s presence, food, the bright sun, and darkness. Finally, even these things fade away.