Scene 1. Madrid, during the heatwave of August 1897, on the eve of the verbena of San Joaquín. Following a Preludio which draws on the main musical themes of the work, the curtain rises on the back parlour of Señor Joaquín's shop. The grocer has two women in his life - his second wife Vicenta, and Trini, his daughter by the previous marriage. Because the two are almost the same age, they are more like sisters than stepmother and stepdaughter. Just now they are nominally engaged in housekeeping, whilst Manuel - the personable young man who does the accounting for Señor Joaquín - busies himself with some paperwork. Trini indulges in some mocking flirtation with Manuel, until Señor Joaquín comes in and levity ceases.
He asks Manuel to hand him an invoice, and is surprised to discover on the back of the paper some amorous doggerel addressed to "a lovely woman". Manuel respectfully denies having written them. In fact the verses have been penned by the gawky, foolish apprentice Chisco, a foundling whom Joaquín has kindly taken in, and who is hopelessly in love with Trini. She treats the whole thing as a joke and goes off to help him and her father in the shop.
Left alone together, Manuel pours out his love to Vicenta, who gently reproaches him and rejects his advances (Dúo: "Vicenta, yo me muero".) It emerges that the two were engaged some years ago, before Vicenta met and fell in love with Señor Joaquín. When her husband surprises the two of them in intimate conversation, he generously concludes that Manuel must be in love with Trini, but afraid to speak for himself - a theory which the terrified young man is quick to go along with. The flustered Vicenta leaves. Joaquín is well pleased by the prospective match - as is Trini when he calls her in to tell her the news, though she shows her high spirits by mocking Manuel even more than usual. Chisco calls "the boss" out to deal with an important customer, leaving Manuel to haltingly declare his love to Trini. She continues to make fun of him in their Dúo: "Quien mi cariño pretenda", a clever sequence of mazurka, tango and zapateado rhythms. Joaquín reappears and calls for Chisco, who is coming out of the cellar carrying a pile of tin cans. On hearing of the "engagement" he tumbles down the stairs in his shock, and the scene ends with the cacophony of the falling cans.
Scene 2. The street in front of the shop. It is dusk, and a Chorus of street sellers are offering their wares - stationery, buttons and hairpins, flowers and azucarillos or meringues. A band of blind beggars join them (Chotis de los ciegos: "Pobrecitos degraciados") before launching into the once-famous Couplets mocking the city authorities (Coplas: "Cachirulipón en las coplas que canto".) A policeman tells them all to move on, and after the crowd has dispersed he tells Joaquín he will return soon to celebrate the shopkeeper's name day. Trini is also outside, getting some fresh air, and Chisco has time to present her with a rose before he is called back in by her father. Before the girl can go back inside, Vicenta appears with Manuel. Trini hides, and overhears the bookkeeper announce his resolve to leave the shop for good. Not only is he giving pain to them both, but now he is in danger of breaking Trini's heart. He feels guilty for using her, and determines not to hurt Trini any more. After the unhappy Vicenta forgives him, he goes back inside to pack his bags. Trini steps forward to confront her stepmother, who explains everything and consoles her, before leading her sadly back into the house. A street band comes in with a crowd, who dance the lively Polka which ends the scene.
Scene 3. The grocers shop, that night. A party is in progress to celebrate Joaquín's name day. Food and drink is served, and Trini takes advantage of the lull to explain to her father that she doesn't love Manuel after all, but Chisco - a story which Joaquín finds hard to credit. When Manuel tells him that he wishes to give his notice and leave immediately, and that his supposed love for Trini was all a mistake, the shopkeeper's jealous suspicions are finally aroused. Vicenta reassures him, explaining that Manuel really wishes to leave because he is ambitious, and work in a shop is no life for him. Joaquín unhesitatingly believes her, and fondly embraces his wife to everyone's satisfaction. It's time for music, and Trini offers to sing an alborada she learned whilst staying in the Galician town of Orense the previous year (Balada y alborada: "Noche pura y serena"). Choosing his moment, Manuel leaves quietly, spotted only by Trini who valiantly keeps singing. After he has gone her strength gives out, and Trini faints into the arms of her stepmother, leaving her song unfinished. The zarzuela ends abruptly with the guests attributing her collapse to the heat, and youthful nerves.