Three hundred years ago, an alchemist by the name of Makropulos, employed by the Hapsburg Emperor, discovered the elixir of life and was forced by the Emperor to administer it to his own daughter, Elina. During the 300 years that she has lived, she has had many identities, many names (always with the initials E.M.), and many affairs. One of her more passionate affairs was with Baron Josef Ferdinand Prus by whom she had a son. She also gave the Baron the formula for the elixir, but the potion killed him and on his deathbed he left his estate to their illegitimate son. Those in attendance around the deathbed, however, misunderstood the name he spoke, and the estate went to a distant branch of the family. The illegitimate branch has contested this for nearly a century.
At Dr. Kolenatý’s law office in Prague, the clerk Vítek is filing papers concerning the long-standing case of Gregor vs. Prus, in which an inheritance has been in dispute for almost 100 years. Gregor, the plaintiff in the case, enters to inquire as to its progress, followed shortly by Kristina, Vítek’s daughter. She is a student of singing and she speaks enthusiastically about the great prima donna, Emilia Marty. Gregor asks Kristina how old Marty is, and she replies that her age is a mystery to everyone.
Dr. Kolenatý enters his office with none other than Emilia Marty. She says that she has come to learn about the Gregor vs. Prus case and displays a remarkable knowledge of it. She claims that Baron “Pepi” Prus, who died apparently childless in 1827, was in fact the father of Ferdinand Gregor, whose mother was the famous singer Ellian MacGregor. (As this could not be proven at the time of the death of Pepi Prus, the Prus estate passed to a cousin.) Marty claims to know of documents in the vaults of the current Baron Prus that will settle the matter conclusively. Dr. Kolenatý does not believe her and cannot imagine how she could know this information, but Gregor insists upon sending him to look into the matter.
After Kolenatý has left, Gregor offers Marty a reward if this information helps him to win his case. She scorns his offer of money but says that she would like a certain document, written in Greek, and that her desire for this document is the reason for her interest in the Gregor vs. Prus case. Kolenatý returns along with Baron Prus, having found Pepi’s will, his love letters to Ellian MacGregor, and a document in Greek. Prus points out that it is still necessary to produce written proof that Ferdinand Gregor was the son of Pepi Prus before Gregor can legally claim the disputed property.
Prus arrives at the opera house, looking for Emilia Marty. Also present in the theater are Kristina, Janek (Kristina’s sweetheart and the son of Prus), as well as Gregor and the old, half-senile Count Hauk-Šendorf. Marty insults Janek and Gregor, and when Hauk-Šendorf remarks on how much she reminds him of his old love 50 years past, the Spanish singer Eugenia Montez, she speak to him in Spanish, leaving him in a confused daze. She turns to Prus, who informs her that he has discovered that Ferdinand Gregor’s mother gave her name as Elina Makropulos at Ferdinand’s baptism. He concludes that, since the love letters to Pepi Prus are only signed with the initials “E.M.,” it cannot be proved that Ellian MacGregor was indeed Ferdinand Gregor’s mother. Marty, desperate for the Greek document in his possession, asks him to name his price for it, but he only laughs and departs.
Gregor makes protestations of love to Marty, but in boredom, she falls asleep. Gregor leaves and Marty awakens to find Janek staring at her in fascination. She charmingly asks him to steal the Greek document from his father, and he agrees. Prus then returns and agrees to give her the document in exchange for a rendezvous.
Marty and Prus are dressing in a hotel room. Marty demands the Greek document from Prus, who throws it on a table. A maid enters and tells Prus that a servant of his is downstairs with a message. Prus hurries out, but quickly returns looking stunned. He tells Marty that his son, Janek, has just killed himself out of love for her. Marty coldly remarks that lots of people kill themselves. Hauk-Šendorf enters and, convinced that Marty is his former lover, begs her to go to Spain with him. Marty agrees, but they are interrupted by the arrival of Dr. Kolenatý, Gregor, Kristina, and Vitek. As Marty is dressing in another room, they search her personal belongings and discover old letters and mementos bearing various names: Emilia Marty, Ellian MacGregor, Eugenia Montez, and Elina Makropulos.
Marty returns in a slightly drunken state; she claims that her real name is Elina Makropulos and that she was born in Crete in 1575. To the disbelief of everyone in the room, she explains that her father, Hieronymus Makropulos, an alchemist to the court of Emperor Rudolf II, devised a longevity potion and administered it to his sixteen-year-old daughter Elina. She has lived ever since then, in different countries under different names, but now feels her strength failing. She requires the formula (the Greek document) if she is to rejuvenate herself. She collapses and is carried to her bed. She revives but appears greatly changed, asking why she ever feared death as it is death that gives life new meaning. She offers the formula to Kristina, and Kristina burns the document. Marty sinks happily into death.