The initial section of the thirteenth-century Tristan en prose has been the subject of multiple critical investigations, mostly devoted to identifying its teleological purpose or figurative function in relation to the rest of the romance. This article proposes to reframe the question by reading the narrative of incest that dominates the “prehistory” of the prose Tristan against two key concepts that inform Michel Foucault’s 1971 essay “Nietzsche, la généalogie, l’histoire.” First, Foucault’s focus on the succession of discursive formations allows for a reinterpretation of the “guerre des récits” that traverses the Tristan: within this critical framework, Foucault’s work on judicial forms permits the identification of the points of emergence of new power/knowledge relationships within the prehistory, with the advent of ecclesiastical authority embodied by Saint Augustine and affirmed in the context of the saint’s public ordeal. Second, Foucault’s reading of the body as the surface of inscription of such discursive conflicts sheds light on the ways in which the saint’s discourse rewrites the bodies of the two protagonists, Apollo and Chelinde, and frames them within the lines of lineage. However, once Augustine’s narrative of “origin” is reinscribed in its discursive context, it clearly appears as purposefully chimeric. Because Tristan’s Arimathean lineage is founded on the revelation of Apollo and Chelinde’s incest, the very idea of unbroken continuity and ordered descent within genealogy is revealed as an illusion: it is a genealogical stemma that only exists in virtue of an archetypal error—the “bad grammar” of incest.