This work studies the behaviour of the protagonist of The History of the Nun: or, The Fair Vow- Breaker (1689) to investigate both what motivates her decisions (from pious woman to murderer) and what the objectives of her author may be. The conclusion reached is that female agency and the transgressive desire developed by Aphra Behn presented a new conception of femininity. Behn challenges the social contract and subverts seventeenth century conceptions of gender. Through Isabella’s behaviour Behn challenges established feminine morality, postulating a new role for women within society. The implication is that the act of breaking a holy vow, and the courage shown by a woman determined to take responsibility for her own life and make her own decisions in society, inevitably ends in tragedy and disaster.