Le Testament français is a partly autobiographical novel which begins and is to a large extent set in Saranza on the edge of the vast Steppes, where the narrator Alyosha spends his childhood and adolescent summers – along with his unnamed sister – with his French grandmother Charlotte. Charlotte speaks to the children in French, although the France she remembers is imbued with a certain glow, a romantic patina which thrills Alyosha, makes him want to understand as much as he can about the country he obviously belongs to, and yet at the same time doesn't know.
The drama of the narrator's double identity – or as he puts it he 'pénible entre-deux-mondes' – his 'painful double-world' – makes up much of the drama in the novel: French is perceived as a 'greffe' ('graft'), 'ma langue grand-maternellel' ('my grand-maternal language'), and his difference can be the subject of discord with his school colleagues, who call him 'Frantsuz', an expression which he views with some ambiguity. The narrator initially, before he attempts to integrate more with his school colleagues, associates with Pashka, the other outsider who doesn't fit in with the norm.
Much of the novel is taken up with the narrator's struggle with his duality, of his acceptance of his double heritage. At the age of thirty we see him in France sleeping rough, almost living the life of a tramp, almost insane, contemplating suicide, and there are little or no explanations for this, apart from his obvious attempts to be granted French nationality, to become an author.
Unable, because unwilling, to re-establish contact with Charlotte in Siberia when living in Paris, the narrator has Alex Bond try to visit her. She was still living at the time, although later an American (via Bond) tells him of Charlotte's death and of his 'testament', by which he means the narrator's legacy, which amounts to a manuscript by Charlotte informing him about the photo (enclosed) that intrigued him at the beginning of the book: the woman saying 'petite pomme' (the French equivalent of 'cheese' to make people appear to smile), who was Charlotte's daughter and the narrator's mother.