Catherine stays at Thrushcross Grange for five weeks. Mrs. Linton spends the time teaching her how to be a proper young lady. Catherine returns around Christmas, wearing a beautiful dress.
Until now, Catherine has been natural, a tomboy, and unaware of class. Her stay at the Linton's changes the first two as she's now a "proper" girl in a dress...
Hindley allows Heathcliff to greet her "like the other servants." Catherine kisses Heathcliff hello, but teases that he's dirty compared to Edgar. Heathcliff walks out, growling that he'll be as dirty as he likes.
...and her comment to Heathcliff makes it clear that she's also been made aware of class, and sees being civilized as superior.
Edgar and Isabella come to Wuthering Heights for Christmas. Heathcliff allows Nelly to make him presentable, but it turns out that Mrs. Linton allowed her children to come only on the condition that they be kept away from Heathcliff. Hindley sends Heathcliff to the kitchen. Before he can go, Edgar makes a disparaging comment about Heathcliff's appearance, and Heathcliff throws applesauce in Edgar's face. Hindley locks Heathcliff in the attic.
The civilized higher classes assert their power by discriminating against the lower class, and they accomplish this discrimination through derogatory language. In contrast, the more "natural" Heathcliff expresses his resentment through violence.
Catherine, though, thinks that both Edgar and Hindley mistreated Heathcliff, and after dinner she slips away from the others to visit Heathcliff. Nelly also takes pity on Heathcliff and brings him down to the kitchen for some food. While eating, Heathcliff tells Nelly that he's going to get revenge against Edgar. Nelly then breaks into her story to say that it is late and she must sleep. Lockwood insists that she continue the story right then.
Catherine's actions show that even though she has been changed by her exposure to civilization, she still shares a yet deeper connection with Heathcliff. Heathcliff's vow is the first sign of his vengeful nature.