Como Desenhar um Círculo Perfeito is a 2009 Portuguese coming-of-age drama directed by Marco Martins in which Guilherme and Sofia, a brother and sister growing up together in a crumbling Lisbon mansion, share an intensely close bond that slowly crosses into forbidden sexual territory as they discover their sexuality together. While Sofia moves on and pursues relationships with other boys, Guilherme becomes increasingly obsessive and possessive — unable to accept her drifting away from him — and will go to dark and disturbing lengths to keep her trapped inside what he calls his own perfect circle, culminating in an incestuous sexual encounter that the film slowly and deliberately builds toward.