ACT I. The Lion’s Mouth. Barnaba, a spy for the Inquisition, contemptuously watches a crowd of Venetians gathered in the Doge’s courtyard for a regatta on the Grand Canal. When the crowd rushes off to watch the race, he reflects on his passion for the beautiful street singer La Gioconda. The singer leads in her blind mother, La Cieca, who blesses her daughter’s kindness (“Figlia, che reggi”). Barnaba tries to force himself on Gioconda, but she escapes him. The crowd returns. Barnaba convinces them that the old lady is a witch who made the popular gondolier Zuane lose the race. The mob threatens the old woman as Gioconda returns with Enzo Grimaldo, a young sea captain she loves, but they are unable to subdue the crowd. Alvise Badoero, the head of the Inquisition, passes with his beautiful wife Laura on their way from a masked ball. They save La Cieca, who, in gratitude, gives Laura her rosary (“Voce di donna o di angelo”). All leave for evening services except Barnaba and Enzo, whom the spy recognizes as a banished Genoese nobleman once in love with Laura (“Enzo Grimaldo, Principe di Santafior”). Barnaba, plotting to prove Enzo faithless to Gioconda, tells Enzo he can arrange for Laura to visit him on his ship that very evening. Enzo cannot resist seeing Laura even though he realizes that Barnaba has evil plans for Gioconda. He hurries to his ship to wait for his lover. Barnaba then dictates a note to the public scribe Isepo telling Alvise of Laura’s impending desertion. He thrusts the note into the Lion’s Mouth (“O monumento”), the repository for denunciations to the Inquisition. Gioconda overhears the plot, and as vespers sound she leans on her mother, crushed by Enzo’s betrayal.
ACT II. The Rosary. Barnaba, disguised as a fisherman, sings a barcarolle (“Pescator, affonda l’esca”) while spying on Enzo’s ship in the Lagoon. He leaves, and Enzo appears on board, waiting for Laura and reflecting on the mystical beauty of sky and sea (“Cielo e mar”). Laura is rowed to the ship by Barnaba, who then disappears with a sneer as the couple embraces. Enzo prepares for the voyage and Laura prays to the Virgin for protection (“Stella del marinar”). Gioconda emerges from the shadows: the two women face off in their rivalry for Enzo’s love (“L’amo come il fulgor”). Gioconda is about to stab Laura when she recognizes her mother’s rosary in Laura’s hand. Realizing it was she who saved La Cieca’s life, Gioconda warns Laura that Alvise is in hot pursuit and sends her rival to safety in her own skiff. Telling Enzo that Laura has left him, Gioconda tries to convince him to join her on the shore, while cannons are fired from Alvise’s ship. Enzo defiantly affirms his love for Laura, sets fire to his own ship, and leaps into the Lagoon.
ACT III. The Ca’ d’Oro. In the Ca’ d’Oro palace, while music is heard in the distance, Alvise vows to himself that his wife will die (“Sì! Morir ella de’!”). Laura enters the room and he orders her to drink a vial of poison before the outside song has ended. As soon as he leaves the room, however, Gioconda slips in and exchanges a sleeping potion for the poison. She will restore her rival to her lover.
Alvise greets his guests in the magnificent ballroom. He presents a ballet for their entertainment (The Dance of the Hours). Barnaba drags in La Cieca, who has been found praying in the house. Enzo, believing Laura dead, reveals his true identity. He addresses her memory; Barnaba gloats while Gioconda offers herself to him if he saves Enzo from Alvise. Barnaba agrees but takes La Cieca as hostage. Goaded by Enzo, Alvise reveals Laura’s moribund body to the shocked and disgusted guests.
ACT IV. The Orfano Canal. Gioconda’s street-singer friends carry the sleeping Laura into Gioconda’s home in a ruined palace on the island of Giudecca. Alone, Gioconda resolves to kill herself (“Suicidio!”). Enzo bursts in, accusing Gioconda of stealing Laura’s dead body. He is about to stab Gioconda when he hears the voice of the awakening Laura. Moved by Gioconda’s generosity and self-sacrifice, Enzo and Laura thank and bless her before leaving on a skiff that Gioconda has provided for them (“Sulle tue mani l’anima”). As soon as Gioconda is alone, Barnaba appears demanding his reward. Stalling for time, Gioconda sings a casual song as she puts on jewels and ornaments. She then takes the dagger and, daring Barnaba to take her body that he wants so badly, stabs herself. Frustrated and irate, Barnaba screams out that he has murdered her mother, but Gioconda can no longer hear him.