Short Answer 
Krogstad blackmails Nora, threatening to expose her past unless she persuades her husband, Helmer, to reinstate him. Nora’s desperation grows as she fears a letter revealing her secret is in the mailbox, while her friend Mrs. Linde’s inability to prevent the situation exacerbates the tension and suspense surrounding the impending revelation.
Step 1: Understanding Krogstad’s Threat
Krogstad is blackmailing Nora by threatening to expose her past illegal loan unless she convinces her husband, Helmer, to reinstate him at work. This creates a tense situation for Nora, who is desperate to keep her secret safe. Krogstad’s leverage over her highlights the precariousness of her situation and the power dynamics at play.
Step 2: Nora’s Desperation
In Act 2, Nora is acutely aware of a letter from Krogstad that is in the mailbox, which she fears will reveal her secret. She pleads with her husband, Helmer, not to read any mail until after the party. However, Helmer dismisses her concerns, believing she is merely being nervous. This misunderstanding escalates the tension between them.
Step 3: Mrs. Linde’s Failure
To complicate matters further, Nora’s friend, Mrs. Linde, is unable to find Krogstad to ask him to retrieve the damaging letter. This inability to intervene means that all signs point to Helmer eventually discovering the truth. The combination of Krogstad’s threat, Nora’s pleas, and Mrs. Linde’s failure heightens the suspense surrounding Helmer’s eventual reaction to the letter.