Ada by Vladimir Nabokov is a novel about the Veen family and an incestuous relationship between two members of the family. The Durmanovs are from Estoty but their favorite residence is in Raduga. Their daughters, Aqua and Marina, are born there. Aqua marries Walter D. Veen, a Manhattan banker who has an affair with Marina. Marina later marries his cousin, also named Walter D. Veen. Marina has two daughters. Years later, two children Pimpernel and Nicolette are rummaging in the attic at Ardis Hall and find mementos of their Aunt Marina. The children are descendants of Princess Sofia Zemske of the Russian Yarolav rulers.

In the summer of his thirteenth year, Ivan, known as Van spends his summer at Ardis Hall with his Uncle Dan, Aunt Marina and cousins Ada and Lucette. His mother, Aqua, was the sister of Marina and his father Demon was a first cousin of Dan. Van is immediately attracted to Ada who is two years younger than he. This summer of 1884 is the first of four consecutive summers that he will spend there. Van remembers, eighty years later, how he fell in love with Ada.

Ada and Van begin kissing and caressing and exploring together and soon become lovers. When he returns to school in fall, he demands that Ada remain faithful to him. He does not return to Ardis Hall until 1888 and they resume their relationship. Van is attending Chose College in England at this time. At this visit, he learns that Ada has been unfaithful to him and walks out of Ardis Hall.

The two lovers lead separate lives for the next four years. Van will not answer any of Ada's letters. When he does, they resume their relationship and are staying at Van's apartment in New York. When his father comes to tell Van of Uncle Dan's death, he finds Ada there. He tells them to come to his apartment and he talks to them separately there. He tells them the terrible truth. They are not cousins as they had thought. They are brother and sister. Demon had an affair with Marina, Ada's mother.

Ada and Van go their separate ways. Ada becomes an actress and marries Andrey Vinelander. Van never marries. He spends most of his time in Europe. He completes his medical training, writes several books and eventually accepts a Chair at the University of Kensington, where he teaches until he retires in 1922. At this time, Ada meets him in Switzerland to settle Lucette's affairs. After Andrey dies a few months later, Ada and Van live together for the rest of their lives. Van begins writing his memoirs at the age of ninety three and he and Ada write several books together. The terrible tragedy of the book is that they love each other and cannot stay away from each other.

The story of their relationship is given in this lengthy novel.