During the excavation of the tomb of an Egyptian priest, two Spanish Egyptologists discover some strange inscriptions which turn out to be a fragment of Critias, Plato's dialogue which narrates the history of Atlantis. Surprised by the find, they embark on an investigation into the origins and significance of their discovery and soon come to rely on the help of a mysterious person who guides their steps. With the help of Álvaro de Andrade, a friend and colleague from the university, they travel to the Brazilian city of Manaus, where they make contact with a retired police inspector who puts them on the trail of an extraordinary city lost in the jungle which could be the key to Critias, the inscriptions in the Egyptian tomb and the existence of a civilisation which disappeared at the dawn of time.