On the steamboat El Dorado, sailing down the Amazon from Leticia, Colombia, to Manaus, Brazil, in the early 1900s.
Act 1
On the riverbank, Riolobo, a mystical character who can assume many forms, announces that the steamboat El Dorado is bound for the opera house in Manaus. There, the legendary opera diva Florencia Grimaldi, who has not set foot in her native South America for twenty years, will give a concert to reopen the theater. Merchants line the riverbank, selling their local wares, while the ship's passengers come aboard: a young journalist, Rosalba, who is working on a biography about Florencia Grimaldi; Paula and Alvaro, a middle-aged married couple journeying to hear Grimaldi in hopes of rekindling the love in their marriage; and the diva herself, traveling incognita.
As the ship pulls away from the busy port, Florencia reflects on the emptiness of her life. She longs to rediscover herself and find her former lover, Cristobal, a butterfly hunter gone in search of the rare Emerald Muse. Rosalba's notebook falls overboard and is rescued from the river by the Captain's nephew, Arcadio. The two exchange confidences about their dreams and desires. Alvaro and Paula attempt to dine on deck, but argue about the exotic menu. Florencia, awakened by the sounds of the jungle, learns from the Captain that Cristobal has disappeared into the jungle without a trace. Later, a tempestuous game of cards reveals the growing affection between Rosalba and Arcadio and the rising tension between Paula and Alvaro.
A violent storm suddenly develops and the ship is carried helplessly in the rushing currents in a downpour of pink rain. Alvaro saves the boat from being wrecked but is knocked overboard. While the Captain is unconscious, Riolobo appears in the guise of a river spirit and implores the mercy of the gods of the river. Arcadio ably takes the helm but the ship runs aground in the maelstrom.
Act 2
In the quiet after the storm, Florencia wonders whether she is alive or dead. Arcadio and Rosalba rejoice to find they have survived the storm but are frightened by the intensity of their feelings for each other. They vow not to fall in love and risk disillusionment. Paula laments the loss of Alvaro, recognizing that the wall between them was pride—not a lack of love.
Riolobo calls upon the mystical and transformative powers of the Amazon. Suddenly, Alvaro reappears, explaining that Paula's voice called him back from the brink of death. On behalf of all the passengers, Florencia thanks him for saving their lives and they resume their journey to Manaus.
Rosalba finds her notebook, containing all her notes for the biography of Florencia, ruined. She is distraught by the loss of two years work, but Florencia tells her she has lost nothing irreplaceable. The two women begin to argue about the source of Grimaldi's talents. When Florencia passionately declares that the diva's gift sprang from her love for another, Rosalba suddenly realizes the woman standing before her is the opera singer herself.
The end of the journey draws near. Suddenly a message comes that cholera has spread in Manaus and no one may disembark. Realizing that she may never find Cristobal, Florencia collapses. Her spirit then drifts towards his in a mystical transformation.