Act I

The opera begins with an invisible choir singing a "Hymn to the Sun," the second best-known piece in the opera.

Iris, the young and innocent daughter of a blind old man, Il Cieco, lives happily, enjoying the sun and the simple things of nature. Osaka, a young lord in search of adventures, plans to kidnap her with the help of Kyoto who keeps a geisha house. During a puppet show, Osaka enters disguised as a child of the sun, singing the serenade "Apri la tua finestra [Open Your Window]," the most famous selection in the opera. He conquers the heart of Iris, and Samurai carry her off, conducting her to Kyoto's geisha house called Yoshiwara. Before leaving, Kyoto anonymously leaves money on Il Cieco's doorstep, as well as a note telling him where she has gone and implying that she has abandoned him.

Act II

At Yoshiwara, where, according to the libretto, the sun never penetrates, Iris wakes up under the illusion of having died and gone to Paradise. Osaka arrives and tries to seduce her but fails to persuade her to yield to his advances. Tired and annoyed by her simplicity and innocence, Osaka tells Kyoto to get rid of her. Instead Kyoto, hoping to make some profit off of Iris, exposes her to the crowds on a balcony of the house. Il Cieco, having assumed that Iris went to the geisha house of her own accord, comes there. He curses her, repeatedly flinging mud in her face. She is overwhelmed by sudden madness at her father's incomprehensible actions. Before anyone, including the remorse-stricken Osaka who has returned, can stop her, Iris rushes back into the house and throws herself down a shaft leading to a sewer.

Act III

The next morning in the sewer, ragpickers begin stealing Iris' silken clothing; she revives, frightening away the ragpickers. She quickly becomes delirious and imagines that she hears the voices of the three men, first Osaka, then Kyoto, and finally Il Cieco, each mocking her. She rejoices when she feels the warm rays of the rising sun, accompanied by a return of the "Hymn to the Sun," and she dies. Tendrils of flowers bear the soul of Iris to heaven.