The plot depends on a tangled set of village relationships. Before the opera begins, the mill-owner Grandmother Buryja's two sons have both married twice, fathered children, and died. Their wives have also died, except for the Kostelnička (widow of the churchwarden), the younger son's second wife and Jenůfa's stepmother. Custom dictates that only Števa, the elder son's child by his first marriage, will inherit the mill, leaving his half-brother Laca and cousin Jenůfa to earn their livings.

ACT I　

Jenůfa, Laca, and Grandmother Buryja wait for Števa to return home. Jenůfa, in love with Števa and secretly pregnant with his child, worries that he may have been drafted into the army. Laca, in love with Jenůfa, expresses bitterness against his half-brother's favored position at home. As he complains he plays with a knife and, finding it blunt, gives it to the mill foreman to be sharpened.
 

The foreman informs the family that Števa has not been drafted, to Jenůfa's relief and Laca's increased frustration. The others leave, and Jenůfa waits to greet Števa. He appears with a group of soldiers, drunk and boasting of his prowess with the girls. He calls for music and drags the miserable Jenůfa into dancing with him.
 

Then Kostelnička steps into this rowdy scene, silences the musicians and, shocked by Števa's behavior, forbids him to marry Jenůfa until he can stay sober for one full year. The soldiers and the family leave Števa and Jenůfa alone, and she begs him to love her, but he, unaware of her pregnancy, gives her casual answers and leaves.
 

Laca returns, as bitter as ever. He attempts to goad Jenůfa into criticizing Števa, but she takes her lover's side despite everything. Laca rages that Števa would never even look at her if it weren't for her rosy cheeks, then slashes her across the cheek with his knife.