Tita's profuse tears cause Rosaura's wedding cake batter to be soggy.
In the story, Tita is Mama Elena's youngest daughter. She has two older sisters, Gertrudis and Rosaura. When a suitor named Pedro comes calling for Tita's hand in marriage, Mama Elena refuses to give her consent. The tyrannical matriarch intends to continue the family tradition of keeping the youngest daughter a virgin. In short, Tita is forbidden to marry; after all, tradition holds that it is the youngest daughter who must care for her parents in their old age.
Instead, Mama Elena prevails upon Pedro to marry Rosaura. Pedro agrees only because he will get to be close to Tita by marrying into the family. Both Nacha (the family cook) and Tita are tasked with cooking the wedding banquet. They have to make enough cake for one hundred and eighty people. Two hundred roosters are ordered for castrating and fattening up (roosters which are castrated and fattened up are known as capons).
Both Nacha and Tita spend three days making twenty different courses for Rosaura and Pedro's wedding. (To punish Tita for feigning a headache to avoid Rosaura's engagement, Mama Elena forces Tita to help Nacha with the food preparations including the castrating of the chickens). In the midst of it, Tita has to pent up her own grief at losing her lover to her sister. Mama Elena, who knows the truth of the situation, is grossly insensitive and unsympathetic to her daughter's suffering.
At last, when Mama Elena leaves the kitchen, the solicitous Nacha tells Tita to let out her frustration and her grief. She is bidden to cry to her heart's content now that her mother has left the kitchen; Nacha doesn't want anyone to see Tita in such a state on the day of the wedding and tells her not to hold back her tears now. So, all Tita's tears pour figuratively and literally into the cake batter; that's why the batter wouldn't thicken.
After Nacha and Tita cry their hearts out, they proceed to finish the rest of the cake for the wedding.