Ada tells the life story of a man named Van Veen, and his lifelong love affair with his sister Ada. They meet when she is eleven (soon to be twelve) and he is fourteen, believing that they are cousins (more precisely: that their fathers are cousins and that their mothers are sisters), and begin a sexual affair. They later discover that Van's father is also Ada's and her mother is also his. The story follows the various interruptions and resumptions of their affair. Both are wealthy, educated, and intelligent. Van goes on to become a world-renowned psychologist, and the book itself takes the form of his memoirs, written when he is in his nineties, punctuated with his own and Ada's marginal notes, and in parts with notes by an unnamed editor, suggesting the manuscript is not complete.The novel is divided into five parts, each shorter in length than the preceding one (Part four being the only exception). As they progress chronologically, this structure evokes a sense of a person reflecting on his own memories, with an adolescence stretching out epically, and many later years simply flashing by. A masterpiece.