The Roland, former Parisian jewellers, have retired to Le Havre, where they live happy days. They have two sons: the eldest, Pierre, in his late thirties, brown, thin and nervous, tormented by big plans and subject to unexpected discouragement, has just obtained his degree in medicine. Jean, her five-year-old, fat, blond, placid younger son, has just graduated from law school. They both came to rest after their successful studies.
They were both considering settling in and coveting the young and beautiful widow of a wealthy captain, Mrs. Rosémilly.
During a family fishing trip with Mrs. Rosémilly, the two brothers, to seduce the young woman, engaged in fierce rowing competition. The reader discovers that under an appearance of union and affection, a real rivalry exists between the two brothers.
That same evening, on their return from this sea trip, they learned that Marshal, an old friend of the family, had just died in Paris and that he was leaving all his fortune to Jean. At the port, Pierre congratulates his brother on his new fortune, but he feels an irrepressible inner feeling of jealousy.
This jealousy, fuelled by the insinuations of the pharmacist Marowsko, to whom he has just informed the news ("it will not look good"), and the thoughts of a girl from the brewery ("it is no wonder he looks so little like you") soon turns into an obsessive suspicion: What if Jean was Marshal's natural son?
Pierre, very shocked, disrupts the happy family meal.
He goes out to sea, but is very affected by the suspicions raised by the remarks of his relatives. He fails to understand why only John inherited Marshal's fortune, even though he had known him since he was a child.
The fog forces him to go home. He then began a "meticulous investigation" to unravel this mystery. He then remembers that when John was born, the portrait of Marshal was taken off the wall.
He asks his mother where the portrait of Marshal is. He lets her know that he knows everything. A trip to Trouville, from which he hopes for a diversion, unfortunately did not change his mind. His mother's attitude towards the portrait, and the similarity he notes between Marshal and his brother, transform doubt into "intolerable certainty".
Unhappy, tortured by this secret and by the remorse that makes him ashamed of himself, Peter torments his mother. On the occasion of a trip to the beach, he declared himself to Mrs Rosémilly, who did not remain indifferent to his advances.
Jean has moved into a new apartment, which makes Pierre jealous. The two brothers quarrel. Peter, in anger, revealed the truth to his brother without hesitation: "You are the son of a man who left you his fortune". Their mother, who is in the next room, certainly hears this horrible confession.
Jean's mother confirmed this secret to him: "You are not Roland's son". She wants to leave forever, but Jean begs her to stay. Mother and son reconcile.
Pierre wanted to leave and decided to embark as a doctor on a transatlantic liner.
Mr. Roland does not suspect the tragedy that is taking place in his family, so in this last chapter we find the same protagonists as at the beginning of the novel. Only Pierre is missing. All this group was gathered in the same boat to greet the departure of La Lorraine, on board which Pierre began a new "life of a vagrant convict".