Katherine of Aragon’s story suffers from knowing how it ends; depictions of her focus on her as a barrier to Anne Boleyn’s disruptive and seductive ascent. Philippa Gregory’s The Constant Princess (2005), however, refuses to pander to familiar stereotypes of Katherine and instead casts her as young, ambitious, and attractive. Gregory refutes the popular contemporary depiction of Katherine of Aragon as “The Betrayed Wife” (Fraser, The Six Wives of Henry VIII [London: Orion Publishing, 1992], 1) by turning her into The Betraying Wife. On his deathbed, Gregory’s Arthur makes Katherine promise to become Queen, to deny that their marriage was consummated, and to progress their Arthurian vision for England by marrying his brother, the future Henry VIII. This chapter examines how Gregory re-imagines Katherine’s place in history and its fictions, exploring how Katherine’s lie positions her as an incestuous figure. Considering the use of her beauty as a site for incestuous threat and opportunity, the construction of her romance with Arthur and their vision for England, and Gregory’s transformation of Katherine’s speech at the Blackfriars court to a literary coda, I explore how the portrayal of Katherine as an incestuous figure limits other opportunities for re-appraisal.