It can be said that Nora's downfall begins to take place when she begins questioning the ethical value of her conduct. Prior to Krogstad raising the issue, Nora had never considered the act of forging her signature to be a serious matter. She felt that she was doing it for the best. She thought that she was saving her dying father from the agony of knowing that his only child's husband was ill as well, and she also thought that she was saving her husband's life. Not only that, in her naivete, she believed that there must be laws that allowed a woman to "spare her dying father" and "save her husband's life" (I). However, now that Krogstad is threatening her with blackmail, she is beginning to realize that what she did was illegal and looked down upon by society.
In addition, immediately after Krogstad's visit, Torvald philosophizes about how every criminal he has come into contact with has had a "deceitful mother" (I). Torvald's speech makes her begin to realize that she herself has been deceitful and that she may be poisoning her home, just as Torvald argues that Krogstad is poisoning his own home and children. We see Nora begin to realize her failings in the final lines of the opening act:
[pale with terror]. Deprave my children? Poison my home? [A short pause. Then she tosses her head.] It's not true. It can't possibly be true" (I).
Therefore, these final lines are particularly useful in portraying Nora's downfall.