"O Primo Basílio" is a novel written by the Portuguese author José Maria de Eça de Queirós, published in 1878. The story revolves around Luísa, a young married woman, who becomes involved in an affair with her cousin, Basílio. The novel explores themes of infidelity, social hypocrisy, and the moral dilemmas faced by its characters. Set in Lisbon, the narrative critiques the bourgeois society of the time, highlighting the struggles of women and the consequences of betrayal. Through its vivid characters and intricate plot, "O Primo Basílio" remains a significant work in Portuguese literature, reflecting the complexities of human relationships.