ACT ONE THE LION'S MOUTH
The scene opens with a carnival in St Mark's Square in preparation for the regatta. In the midst of the festivities is Barnaba, a spy for the Inquisition, dressed as a street singer.
( The inquisition was a tribunal established by the Roman Catholic church, originally in the 13th century to suppress heresy. Punishments included imprisonment, flogging and death by burning. )
Barnaba is in love with La Giaconda, a street singer, who now appears with her blind mother, La Cieca. Barnaba's advances are spurned by La Giaconda, and he swears revenge.
The company reappears after witnessing the regatta and Barnaba wins the confidence of the gondolier Zuare who has suffered a humiliating defeat at the regatta. He tells Zuare that his case was hopeless before he even entered the race because he had been cursed by La Cieca. Urged by Barnaba, the crowd takes up Zuare's cause & calls for the old woman to be burnt at the stake as a witch.
Just as matters reach a climax, Enzo Grimaldo, a prince from Genoa, and La Giaconda's lover arrives to intervene. He has been banished from Venice by the State Inquisitor, Alvise Badoero, but has secretly returned disguised as a Dalmatian sea captain.
At that moment, Alvise and his Genoese wife Laura arrive home from a masked ball. Laura begs her husband to show mercy on the blind old woman whose piety they recognise in the rosary she carries. On being set free by Alvise, La Cieca gives her rosary to his wife, Laura as a mark of her gratitude.
By this time, however, Barnaba has recognised Enzo & confronts him, calling him by his true name. Barnaba reveals to Enzo his influence with the High Council which fills Enzo with apprehension. However Barnaba also reveals that he can arrange for Enzo to meet his childhood sweetheart, Laura, on board Enzo's ship that night. ( Laura had been forced to marry Alvise against her will. )
Knowing that Giaconda is in love with Enzo, Barnaba hopes that this action would deeply wound her. Enzo is now torn between his love for Laura, & his fear that Giaconda will come to harm; but his love for Laura wins. He curses Barnaba for his cruelty and leaves. Barnaba immediately dictates a letter to the scribe Isepo, which informs the Inquisitor of Enzo's secret meeting with Alvise'e wife, Laura that night on board the ship. He drops the letter into the Lion's Mouth into which all anonymous denunciations are left for the Inquisitor.
The crowds reappear, still in carnival mood & dance the Furlana: the dance is interrupted by the Angelus & all kneel for prayer.
During the prayers Giaconda reappears. She has overheard the plot between Barnaba and Enzo and is bitterly upset that Enzo should betray her for Laura.
ACT TWO THE ROSARY
It is evening and the sailors are preparing Enzo's ship to set sail under the watchful eye of Barnaba, now dressed as a sailor. Enzo, the captain mow appears on board and prepares the ship for sailing, but, on seeing the sky in all its majesty, is unable to contain his feelings of expectation and love.. At a prepared signal from Barnaba, he helps Laura on to the quay and takes her in his arms. They plan am elopement, and while Laura. prays to the Virgin to protect them, Enzo gives the ship a final check before departure. Suddenly Giaconda appears, ready to fight for Enzo's affection. She is on the point of stabbing Laura when shy recognises her mother's rosary in Laura's hand.
The rosary saves Laura's life, and in a wave of spontaneous compassion, Giaconda warns Laura that he husband Alvise is fast approaching, and helps Laura to escape. Giaconda tells Enzo however, that Laura ha gone because she has discovered that she mot longer loves him. Enzo scorns Giaconda's declaration and advances. She warns him that Barnaba has denounced him to the Inquisition and that he has only a short time to live. In desperation Enzo sets fire to his ship and, with the name of Laura on his lips escapes into the lagoon rather than surrender to Alvise and the Inquisition.
ACT THREE SCENE ONE A CHAMBER IN ALVISE'S PALACE
In his palace beside the Grand Canal, Alvise is furious and has Laura thrown into a chamber specially prepared for her death. He leaves for her a vial of poison with which she is to commit suicide. But Giaconda has secretly broken into the palace and, out of love for Enzo, she once again saves Laura by slipping her a sleeping draft with which she can feign her suicide.
SCENE TWO THE BALLROOM
In the ballroom Alvise greets his guests, offering them as entertainment a performance of the ballet The Dance of the Hours. On completion of the ballet Barnaba enters dragging La Cieca, whom he has found inside the palace. She insists that she had merely been praying for the dying. Enzo has entered and enquires of Barnaba who is dead. On being told that it is Laura, Enzo reveals his true identity to Alvise, and demands satisfaction for the theft of his home and his beloved.
Alvise triumphantly orders the attendants to bring the bier carrying Laura's body to be brought in - he states that she had besmirched his honour. The guards move in to arrest Enzo and, in the ensuing scuffle, Giaconda promises herself to Barnaba if he succeeds in saving Enzo.
ACT FOUR A CHAMBER IN A RUINED CASTLE
In a chamber of a ruined castle which serves as Giaconda's home, Giaconda receives Laura's body. In a desperate turmoil Giaconda holds the vial of poison which Alvise had given to Laura. Now that she once again has Laura in her power, should she get rid of her in the canal, or should she take the poison herself. In the end her love for Enzo leaves her only one option, - suicide.
Enzo, having been released from captivity arrives, and Giaconda greets him warmly. But when he shows only love and remorse for Laura, she invents the lie that she had stolen the corpse, in the hope that his fury will be roused enough to kill her.
At that moment Laura wakes up and tells Enzo of Giaconda's selflessness in saving her. Laura arranges for the two to escape.
Barnaba appears to claim his part of the bargain; in desperation Giaconda stabs herself. The monstrous Barnaba calls out that he has murdered her mother, but it is too late - oblivious to his cries, La Giaconda sinks to the ground.