Act 1
Scene: The entrance hall of the temple of Diana in Tauris.

There is no overture; the opera begins with a short passage evoking calm before turning into a depiction of a great storm at sea. Iphigenia, sister of Orestes, is the high priestess of Diana in the temple of Tauris, having been transported there magically by the goddess when her father Agamemnon attempted to offer her as a sacrifice. Iphigenia and her priestesses beg the gods to protect them from the storm (Grands dieux! soyez nous secourables).

Although the storm dies down, Iphigenia remains troubled by a dream she has had, in which she envisioned her mother Clytaemnestra murdering her father, then her brother Orestes killing her mother, and finally her own hand stabbing her brother. She prays to Diana to reunite her with Orestes (Ô toi qui prolongeas mes jours). Thoas, King of Tauris, enters. He too is obsessed with dark thoughts (De noirs pressentiments): the oracles, he tells her, predict doom for him if a single stranger escapes with his life. (The custom of the Scythians, who inhabit Tauris, is to ritually sacrifice any who are shipwrecked on their shores).

A chorus of Scythians comes bringing news of two young Greeks who have just been found shipwrecked, demanding their blood (Il nous fallait du sang). After Iphigenia and the priestesses depart, Thoas brings in the Greeks, who turn out to be Orestes and his friend Pylades. After asking them for what purpose they came (they have come to retrieve Diana's statue and return it to Greece, though they do not divulge this), Thoas promises them death and has them taken away.

Act 2
Scene: An inner chamber of the temple

Orestes and Pylades languish in chains. Orestes berates himself for causing the death of his dear friend (Dieux qui me poursuivez), but Pylades assures him that he does not feel dispirited because they will die united (Unis dès la plus tendre enfance). A minister of the sanctuary comes to remove Pylades. Orestes half falls asleep (Le calme rentre dans mon coeur), but he is tormented by visions of the Furies, who wish to avenge his slaying of his mother (whom Orestes killed for murdering his father Agamemnon).

Iphigenia enters and, although the two do not recognize each other, Orestes sees an astonishing likeness between her and the slain Clytaemnestra seen in his dream. She questions him further, asking him the fate of Agamemnon and all Greece, and he tells her of Agamemnon's murder by his wife, and the wife's murder by her son. In agitation, she asks of the fate of the son, and Orestes says that the son found the death he had long sought, and that only their sister Electra remains alive. Iphigenia sends Orestes away and with her priestesses laments the destruction of her country and the supposed death of her brother (Ô malheureuse Iphigénie). She and the priestesses perform a funeral ceremony for Orestes (Contemplez ces tristes apprêts).

Act 3
Scene: Iphigenia's chamber

Iphigenia is drawn to the stranger who reminds her of her brother Orestes (D'une image, hélas! trop chérie). She tells Orestes and Pylades she can persuade Thoas to save one of them from the sacrifice (Je pourrais du tyran tromper la barbarie) and asks the one who is spared to carry word of her fate to her sister Electra in Argos. Both men readily agree, and Iphigenia chooses Orestes to survive.

But on her exit, Orestes insists that Pylades agree to switch places with him as Orestes cannot bear the thought of his friend's death and sees dying as an escape from his own madness; Pylades, on the contrary, is glad at the thought of dying so Orestes can live (Duet: Et tu prétends encore que tu m'aimes and aria for Pylades: Ah! mon ami, j'implore ta pitié!). When Iphigenia returns, Orestes insists that she reverse her decision, threatening to kill himself before her eyes if she does not. Reluctantly, she agrees to spare Pylades instead and sends him to carry her message to Electra. Everyone but Pylades departs, and he closes the act by promising to do everything possible to save Orestes (Divinité des grandes âmes!).

Act 4
Scene: Inside the temple of Diana

Iphigenia wonders how she can ever carry out the killing of Orestes, since somehow her soul shrinks from the thought of it. She asks the goddess Diana to help her steel herself for the task (Je t'implore et je tremble). The priestesses bring in Orestes, who has been prepared for sacrifice (Chorus: Ô Diane, sois nous propice). He tells her not to lament him, but to strike, telling her it is the will of the gods. The priestesses sing a hymn to Diana as they lead Orestes to the altar (Chorus: Chaste fille de Latone). While she wields the knife, Orestes exclaims Iphigenia's name, leading her and the priestesses to recognize him and stop the ritual slaughter.

The happy reunion of sister and brother is cut short at news that Thoas is coming, having heard that one of the captives was released and intent on the blood of the other. The king enters wildly, ordering his guards to seize Orestes and promising to sacrifice both him and his sister. At that moment Pylades enters with a band of Greeks, cutting down Thoas where he stands.

The resulting rout of the Scythians by the Greeks is halted by a dea ex machina appearance of Diana, who commands the Scythians to restore her statue to Greece (Arrêtez! Écoutez mes décrets éternels). She also issues pardon to Orestes for murdering his mother, sending him to be king over Mycenae and bidding him restore Iphigenia to her country. As Diana is carried back into the clouds, everyone sings a concluding chorus of rejoicing at having the favor of earth and heaven restored to them (Les dieux, longtemps en courroux).