The Housekeeper is assigned to work for a genius mathematician (the Professor) who, due to an accident some 30 years before, has a memory that lasts only 80 minutes since the accident. She begins to take care of him and his house, and immediately discovers that he loves to constantly talk about math and numbers. Prime numbers in particular. In order to help get himself through the day, the professor pins notes to his suit, such as "You're memory only lasts 80 minutes".

When the Professor discovers that the Housekeeper has a son, he insists that he come to his house every night for dinner because it is not right for a boy to be away from his mother so much. She concedes and Root (a name given to her son because his head is flat like a square root sign) begins to come. The Professor loves Root and begins helping him with math and listening to baseball games on the radio.

During their time together, the Professor teaches them to love mathematics the way he does. Even though they do not possess his vast knowledge of the subject, his skill as a teacher encourages them to always be thinking about and intuiting answers about mathematics.

One night they take him to a baseball game and the Professor leans over Root to protect him from a foul ball. After the game, the Housekeeper realizes the Professor has a fever so she stays the night to help him through it. It takes some time for him to recover, but he is soon back to full health. The Housekeeper, however, gets fired from her job when it is discovered she stayed longer than she was meant to.

Not long after starting a new position, the Housekeeper is called into her boss's office because Root went to visit the Professor. The widow (the Professor's sister-in-law who paid to have a Housekeeper) thinks Root is there at the Housekeeper's bidding, but she realizes she was mistaken when the Housekeeper and Root claim to be the Professor's friends. The Professor writes a formula on a piece of paper (Euler's formula) in order to end the arguing between the Housekeeper and the Widow.

The Housekeeper is reinstated at the Professor's. She discovers a note in an old thesis paper written in the Professor's hand that says "For N, with my eternal love. Never forget." He wins a prize for solving a puzzle in the Journal of Mathematics, and they decide to celebrate it along with Root's 11th birthday. Before the party the Professor begins to exhibit signs that his memory no longer lasts even 80 minutes. At the party, he forgets who the Housekeeper and Root are after only 10 minutes. Soon after, the Widow puts him into an assisted living home. His memory no longer lasts beyond the accident, not even 80 minutes like befor.

Over the years the Housekeeper and Root continue to visit the Professor in his new home. Root becomes a math teacher after college, and the Professor passes away around 11 years later.