The codicological study of a medieval manuscript is often confined to the simple formal description of the document. Observing the material support of the document with historical, literary, and linguistic data may lead to a genetic study of the text through the analysis of the volume as a whole. Analyzing the ms. Paris, BnF, fr. 12538 from this perspective permits us to establish the presumed history of the Mystère de saint Vincent of which it remains the sole witness. While the irregularities in the organization of the notebooks and their foliotation instruct us on the manuscript’s composition and the writing of the text itself, the use of auxiliary signs – brackets, insertion marks, crosses, and boxes – and the presence of empty spaces for missing portions of the text – words, verses, passages, replies – immerses us in the work of the fatiste. The conclusion of this codicological and genetic study leads us to believe that the mystery conserved in the ms. BnF, fr. 12538 is not that which was presented in Angers in 1467 before King René, but this same text reworked in light of a presentation that could have taken place in Le Lude in 1476.