Adina, wealthy owner of a business, her friend Giannetta and a group of workers are resting. At a distance Nemorino, a young villager, laments he has nothing to offer Adina but love. The workers urge their mistress to read them a story - how Tristan won the heart of Isolde by drinking a magic love potion.
No sooner has Adina done so than Belcore swaggers in with his troop. The soldier's conceit amuses her, but he is not dissuaded from asking her hand in marriage, confident that no girl can resist a soldier. She declines the offer, and the jealous Nemorino wishes he could approach her as confidently as Belcore. Adina offers wine to Belcore and his comrades. Nemorino plucks up the courage to speak to Adina of his love, but she answers that she is capricious and wishes to remain free, advising him to go to the city and live with his rich, ailing uncle. He is unable to accept her advice to love lightly as she does.
Heralded by a trumpet, the traveling salesman Dr. Dulcamara arrives, promising potions to cure all ills. Since his medicine is inexpensive, the workers buy eagerly. When they have gone, Nemorino asks Dulcamara if he sells Isolde's elixir of love described in Adina's book. Dulcamara, at first puzzled, recovers and pretends to have just the thing. Pulling out a bottle of Bordeaux, the charlatan declares this is the very draught and sells it to Nemorino - with instructions to drink it slowly. Though it costs him his last cent, Nemorino buys what he belives to be the elixir of love and hastily drinks it.
Adina enters to find him tipsy. Certain of winning her love, he pretends indifference. To punish him, Adina flirts with Belcore, who, informed that he must return to his garrison, persuades her to marry him at once. Horrified, Nemorino begs Adina to wait one more day, but she ignores him and invites the entire village to her wedding feast. Nemorino rushes away, moaning that he has been ruined by Dulcamara's elixir.
The pre-wedding supper is in progress. Everyone except Nemorino is celebrating. Dulcamara, self-appointed master of ceremonies, produces a new duet which he sings with Adina. It tells of a girl who refuses a rich suitor because she loves a poor man. When the time comes for the signing of the contract, Adina is angry that Nemorino is not present. The guests disperse. Remaining behind, Dulcamara is joined by Nemorino, who begs for another bottle of elixir explaining that he cannot wait till the next day for the elixir to take effect. His pleas are rejected, because he has no money. Belcore returns, annoyed that Adina has postponed the wedding until nightfall. When he learns of Nemorino's problem, he explains to the youth that he will receive cash immediately if he enlists. Nemorino does so, to the amusement of Belcore at having enlisted his rival.
Adina's friend Giannetta tells the girls that Nemorino's uncle has died and left him a large fortune. When the youth reels in, giddy from a second bottle of wine, they besiege him with attention. Unaware of his new wealth, he believes the elixir finally has taken effect.
Adina dislikes his apparent desertion of her, while Dulcamara wonders if he has accidentally hit on the real elixir of love. When he explains to her that Nemorino had bought the elixir from him to win the heart of some unfeeling beauty, Adina realises that Nemorino really loves her and is remorseful at her treatment of him. Perceiving that she loves Nemorino, Dulcamara decides that she needs a dose of the potion to bring him back to her, but she assures him that she needs no elixir other than her own beauty, which Nemorino is powerless to resist.
Nemorino has seen a tear in Adina's eyes and believes she will soon be his, but pretends to be indifferent, telling her he is unable to choose between the village girls clamoring for his attention. She tells him that she has bought back his freedom from the army because he is needed in the village, but is reluctant to admit to her love until he announces that he will still become a soldier if she does not.
Belcore marches in to find Adina affianced to Nemorino. But he soon recovers from his rejection, declaring that thousands of women await him. Dulcamara points to the success of his elixir and is besieged by eager customers before making his escape.