The action takes place at the port of Leticia, Colombia and then on the riverboat El Dorado.
Riolobo, a character who can assume various forms, announces that the El Dorado is setting off down the Amazon for the opera house in Manaus where the legendary diva Florencia Grimaldi, who has not set foot in her native South America for 20 years, will give a concert. Florencia arrives on the dock incognito to make the river journey. Her motive for this trip is to search for her long-lost lover, Cristóbal Ribeiro da Silva. Twenty years ago, they parted on the river when he went in search of the world's rarest butterfly, the Emerald Muse, and Florencia set out to pursue her life as an opera diva in Europe. Among the other travelers are the ship's Captain, his nephew Arcadio, the young journalist Rosalba, who is working on a biography about the famous singer, and Paula and Alvaro, a middle-aged couple journeying to hear Grimaldi in hopes of rekindling their marriage.
At dawn as the ship leaves behind the busy port, Florencia reflects on her history and her motives for making the trip. Arcadio and Rosalba grow closer as they exchange confidences. Alvaro and Paula attempt to dine on deck, but exchange only bitter words. Later a card game takes place between the two couples that underlines their differences. Florencia passes a sleepless night and then learns from the Captain that the butterfly hunter has not been seen for many years. Suddenly a storm of pink rain develops. Riolobo calls upon the gods of the river; the injured Captain calls for help. Alvaro falls overboard as Arcadio takes the helm, and the ship runs aground.
In the quiet after the storm Florencia awakens, wondering whether she is alive or dead. Arcadio and Rosalba rejoice to find they have survived the storm. Paula sees Alvaro's body and laments his death. Suddenly, he awakens and the passengers resume their journey to Manaus. Florencia and Rosalba argue over the source of the opera singer's talents. Florencia argues so persuasively that the writer realizes the woman standing before her is the diva herself. It is the first of other epiphanies.
The passengers anticipate the end of their journey as the ship's arrival in Manaus is announced. But suddenly a message comes that cholera has spread in Manaus and no one may disembark. As Florencia collapses, realizing she may never find Cristóbal, her spirit drifts towards his in a mystical transformation.