A perfect little novel. A housekeeper goes to work for a math professor who, due to a previous head injury, can only remember things for 80 minutes. He can remember everything up to his head injury, but everything that happens after that can be remembered only for 80 minutes, and then it disappears as if it had never happened. He covers his clothes in post-it notes that serve as reminders for him. The most important one says "my memory is only eighty minutes long" and he wears this on his sleeve so that he always knows the reason why he is confused. He is a brilliant mathematician, and that brilliance is still preserved. He has an autistic-like familiarity with numbers and their relationships, and he imbues them with life and personality, and conveys this enthusiasm to his housekeeper and her young 10 year old son Root (nicknamed by the professor, for the square flat top of his head). Every time the housekeeper comes to work, she has to introduce herself to the professor. But they come to form a warm and generous friendship. She accepts his disability and works with it, and doesn't regard him in a lesser light for it. He opens up to her a new world of mathematics and shows her the beauty of it. The book is peppered with complex mathematical theories and yet they don't intrude. The author has presented them to us in the same way as the professor, so in a way she has become the professor, and we the housekeeper.
Despite the content, the story is never heavy or cumbersome. It always remains airy, evenly paced and quiet, very "Japanese".