The Young Syrian, the Page of Heroidias, and some soldiers are gathered on a balcony at King Herod's palace, overlooking a walled cistern (a well). Offstage, Herod is conducting a banquet with many international guests. While the Syrian and the Page discuss the moon, a figure for the princess Salome with whom the Syrian is infatuated, four soldiers discuss a group of Jews in Herod's court, who are loudly "disputing their religion." They become anxious that the Tetrarch is "sombre" because he is looking at someone, and they try to determine who it is. They begin a conversation about religion, which is interrupted by an offstage voice shouting prophesies about the coming of the Messiah. One of the soldiers reveals that it is the voice of Iokanaan, a gentle prophet whom Herod had captured in the desert alongside his sizable following of disciples. The prophet is now imprisoned in the palace cistern, which one of the soldiers reveals is the same place Herod had his brother (who was his wife Herodias's husband at the time) imprisoned for twelve years before he had him strangled. When one of the men asks to see the prophet, another soldier explains that Herod has forbid anyone from visiting or even looking at his prisoner.
Salome enters, explaining to the group on the balcony that she grew tired of the banquet, during which her step-father Herod was constantly staring at her. Upon hearing the shouting of Iokanaan, Salome becomes curious. She asks the Young Syrian and the Second Soldier questions about the body behind the voice. They explain that Iokanaan is the prophet who has been shouting "terrible things" about Herodias, Salome's mother, and tell her that he is imprisoned in the palace cistern. After dismissing the slave who comes to bring her back to the banquet, Salome convinces the Syrian to let her see the prophet by promising him that she will look at him the next day when she is traveling through the gateway of the idol-sellers.
Salome, finding that she is attracted to Iokanaan, begs him for a kiss, but he refuses, recognizing her as the daughter of Herodias, whose incestuous marriage he considers blasphemous. He refuses to look at her. Upon seeing Salome's desire for Iokanaan, the Syrian becomes distraught and kills himself. Two soldiers rush to hide his body from Herod while the Page, infatuated with the Syrian, mourns the death of his dear friend. Iokanaan, urging Salome to seek "the Son of Man," returns to the depths of the cistern.
Herod and Herodias enter, accompanied by their guests. Aware that Salome is nearby, Herod, to his wife's disapproval, asks his slaves that the festivities be moved outside. Herod slips in the blood of the slain Syrian, which he recognizes as an omen. Nevertheless, Herod soon abandons the conversation he is having with his guests about religion and politics to try to persuade Salome to join him over wine and fruit. She refuses, which amuses Herodias. The voice of Iokanaan, shouting insults about Herodias, interrupts the discussion and upsets the queen. She suggests to her husband that he turn Iokanaan over to the Jews who "have been clamoring for him." A conversation between five Jews reveals that, while they have different thoughts about the "unseen" Jewish God, they are all offended by Iokanaan's claim to be such a prophet--an honor conferred only on the Hebrew prophet Elias.
After discussing the controversial figure of Christ, to whom he is indifferent, with the Jews and a Nazarene, Herod is again distracted by the sight of Salome. Transfixed, he asks her to dance for him. When she refuses him, Herod offers her anything of her choice from his vast kingdom for a single dance. Ignoring her mother's orders that she not dance, Salome finally agrees after Herod has sworn on his life that he will grant her anything she desires.
Despite the ill omen of blood on the floor, and Iokanaan's shouting prophesy of doom, Herod allows Salome to perform the Dance of the Seven Veils. After she has finished, she asks him for Iokanaan's head on a silver charger. Herodias, believing that Salome is avenging her honor, is pleased with the request, but the rest of the court is scandalized. Herod pleads with Salome to ignore her mother's wishes, but Salome assures the Tetrarch that she is acting on her own desires. After many attempts to bargain with Salome fail, Herod reluctantly orders the execution of Iokanaan. While the executioner hesitates to complete the task, he finally beheads the prophet inside the cistern at the urging of soldiers under Salome's command. The executioner presents Iokanaan's head to Salome on his silver shield. After initial joy, Salome regrets that Iokanaan's eyes are still closed, indicating that, even in death, he refuses to return her lustful gaze.
Certain that "some terrible thing" will befall his court, Herod orders his slaves to "put out the torches" and "hide the moon" and then prepares to leave the terrace for the relative safety of the palace interior. As Herod's attendants extinguish the terrace torches, a cloud suddenly conceals the moon, leaving only a ray of light to illumine Salome as she finally kisses Iokanaan on the mouth. Catching sight of the kiss on his way up the palace staircase, Herod orders his soldiers to "kill that woman," so Salome is crushed to death beneath their shields.