Ibrahim, a prince from the Komnenoi dynasty allied to the Greek imperial family and converted to Islam after being kidnapped by the Turks, becomes a favourite of Mahomet second. The sultan sent him to fellow prisoner Greek princess Irène to tell her that her father and brother are alive, and that he intends on marrying her. She refuses the sultan's offer and despises Ibrahim for submitting himself to the Turks. Mahomet, who wanted to marry his sister to Ibrahim, then decides to wed her to Lascaris, and expects Irène to be grateful and accept to marry him. He frees Théodore and Lascaris in front of Irène, and the act ends with Roxane and Lascaris professing love for each other.