The true story of a secret romance that changed the course of modern art—told in full for the first time. “Tell me who your enemies are, so that I can help you to hate them.” This was a favourite line of the Brazilian sculptor Maria Martins (1894–1973), and likely how she greeted Marcel Duchamp (1887–1968) at their first meeting, at the opening of an exhibition of her work in 1943.The clandestine love affair that burned between these two artists for the remainder of the decade was fuelled not only by romantic passion but also by a deep appreciation of each other’s work. Some of the greatest of Martins’s Surrealist sculptures were inspired by her relationship with Duchamp—including Impossible, whose title perhaps refers to their situation. (Martins was married to the Brazilian ambassador to the United States.) And it was during their affair that Duchamp—widely believed to have “retired” from art—secretly began his astonishing final work, which would be revealed only after his death. This work, the sculptural tableau Étant donnés, was heavily influenced by Martins: it takes formal cues from her work, and it was she who modelled for the startlingly realistic nude at its centre. This surprising story—highly readable yet meticulously researched, and abundantly illustrated with artworks and archival images—will affirm the importance of Maria Martins’s tropically inflected Surrealism and challenge the perception of Duchamp as a purely cool and cerebral artist.