The twin of the title is Helmer van Wonderen. He is a 54 year old dairy farmer in Noord-Holland, the northwest peninsula of the Netherlands and the year is 2002, 35 years after his only sibling, his twin brother Henk, died. Henk was the front seat passenger of a car driven by Henk's fiancée, Riet, when the car plunged into lake Ijssel. Riet lived; Henk drowned. Helmer's life immediately changed from that of a 19 year old university student in Dutch linguistics to a farmer and successor to their father, a tyrannical and distinctly unlovable man. Henk had been the father's clear favorite, if we accept Helmer's and the narrator's viewpoints. Helmer stays a bachelor and maintains the farm into the present, the time of the novel. His father is elderly and confined to the upstairs. Helmer treats him with disdain; he feeds and bathes him with barely disguised contempt awaiting his death with a vague sense of hope, symbolized by Helmer's re-organizing and painting the interior of their house at the beginning of the book.

Abruptly Riet, a recently widowed mother of three, re-enters the van Wonderen world with a letter requesting Helmer to allow her youngest child, an 18 year old son, also named Henk, to live with Helmer as a farmhand. Riet wants her son Henk to learn farming and discipline and receive the parental (read "fatherly") direction she feels he needs and she cannot supply. Helmer consents. Henk comes to live with him, working desultorily as a hired hand.

Riet and Helmer become estranged over the latter's lying to her that his father was dead when in fact he was upstairs at the time of her only visit to their home since her fiancé's death. One day Henk saves Helmer's life when the latter becomes pinned by a sheep in a ditch. Henk leaves soon thereafter; the father dies; and a Frisian farmhand from Helmer's youth re-appears at the time of the father's funeral. He and Helmer take off, after Helmer sells the farm, to Denmark, a much vaunted Shangri-La for Dutch farmers in this novel.