Lord Killarney, delegate of the British Museum, acquires in Córdoba, at an auction, an incunabule, a valuable treatise on Arabic botany, but he is killed and the book disappears. The suspicions fall on El Lince, an old-fashioned antiquarian who had bid with the English to get the book. When he is arrested, he asks his nephew to warn Curro Jiménez, who discovers that the famous treaty actually contains the key to accessing a fantastic treasure that a caliph has hidden in the Mosque of Córdoba and that has been, for centuries, cause of deaths and betrayals. But the stolen book is split in half. The other half is, for years, in the power of El Lince, which he bought from the famous bandit Mariano Romero, who has been presumed dead for thirty years.