The climax of A Doll's House is in Act III, shortly after the party, and takes place after Torvald realizes that one of his best friends, Dr. Rank, has shut himself away to die.
Torvald seems genuinely affected by his friend's impending death, and reading the letter from Krogstad distresses him completely. Upon reading about the debt that Nora took on, Torvald instantly grows angry and stops his wife from leaving the house as she was starting to do. Idealistically, Nora thinks that Torvald will take the blame for her forgery and intends to commit suicide to free Torvald from her guilt.
"Nora. Let me go. You shall not suffer for my sake. You shall not take it upon yourself.
Helmer. No tragedy airs, please. (Locks the hall door.) Here you shall stay and give me an explanation."
Nora begins to understand that Torvald is not nearly as noble as she thought he was. Nora has complete faith in her past actions and feels no guilt for doing what was necessary, but only for how it is affecting her husband. However, when she realizes that Torvald hates her for her crime, Nora understands that her husband does not truly love her, does not respect her, and will disavow her for saving his life.
Torvald insists that they must capitulate to Krogstad's wishes and hthat he will no longer allow her to raise their children because she is immoral and a liar. Before he can continue, a new letter arrives from Krogstad saying that he no longer has any intention of blackmailing them.
Torvald's emotions instantly change and he begins to rejoice and is no longer angry with her. He begins to act she'd never be injured by such harsh and cruel words and starts talking to her lovingly again. He forgives her and says that her actions were because of "womanly helplessness," nothing more.
Nora says very little to him, only that she is "beginning to understand thoroughly"; she changes out of her party dress, then tells him to sit down. She begins to tell him that they have never understood each other during their marriage and that he has wronged her terribly. This conversation, about how little Torvald respects her and how unhappy she is, leads to Nora announcing her choice to leave Torvald for good (her decision was already made well before and that is why she changed into her day clothes, ready to walk right out the door).
The climax of any story is often thought to be a point near the end of the story when the tension, or crisis, is at its highest. The most stressful and important part of A Doll's House is when Torvald reads the letter leading Nora to finally realize how manipulated her marriage is.