São Tomé, 1591. Luzia Correia, a Black Portuguese chartmaker who trusts nothing she cannot measure, takes a Crown commission to chart the island's unknown southern coast. Her late master filled that blank with a sea serpent she always took for a lazy man's ink, and her patron, a sugar lord, has plans of his own for her chart. A historical voyage of instruments, interpreters and one marvel that refuses to stay in the margins.