Torvald's mailbox.

Nora confides her dire situation to Kristine Linde, Nora's childhood friend, who was long ago romantically involved with Nils Krogstad.

Mrs. Linde is determined to have Krogstad retrieve the letter before Torvald reads it.

NORA. The letter is lying there in the box.

MRS. LINDE. And your husband keeps the key?

NORA. Yes, always.

MRS. LINDE. Krogstad must ask for his letter back unread, he must find some pretence—

NORA. But it is just at this time that Torvald generally—

MRS. LINDE. You must delay him. Go in to him in the meantime. I will come back as soon as I can.

Mrs. Linde returns a short time later to tell Nora that Krogstad is out of town until the next day, but she will speak with him when he returns. In the meantime, Nora is to keep Torvald from reading the letter until Mrs. Linde has an opportunity to talk with Krogstad.

In act 3, Mrs. Linde meets with Krogstad; they renew their former romantic relationship, and Krogstad offers to retrieve the letter he wrote to Torvald.

KROGSTAD. I will ask for my letter back.

MRS. LINDE. No, no.

KROGSTAD. Yes, of course I will. I will wait here till Helmer comes; I will tell him he must give me my letter back—that it only concerns my dismissal—that he is not to read it—

MRS. LINDE. No, Nils, you must not recall your letter.

Krogstad doesn't understand why Mrs. Linde doesn't want him to retrieve the letter, since it will cause considerable difficulty for her friend Nora.

KROGSTAD. But, tell me, wasn't it for that very purpose that you asked me to meet you here?

MRS. LINDE. In my first moment of fright, it was. But twenty-four hours have elapsed since then, and in that time I have witnessed incredible things in this house. Helmer must know all about it. This unhappy secret must be disclosed; they must have a complete understanding between them, which is impossible with all this concealment and falsehood going on.

Mrs. Linde believes that the only way for Nora and Torvald to continue in their marriage is for Torvald to know the truth.

Mrs. Linde's good intentions fail to produce the desired effect. Torvald reads Krogstad's letter, but instead of acting on Nora's behalf to defend her and then to forgive her for doing what she believed she needed to do to save his life, Torvald decides to accede to Krogstad's demands to avoid damage to his own reputation.

In the midst of angrily berating Nora for putting him into this situation, Torvald intercepts a letter from Krogstad intended for Nora, in which Krogstad writes that he no longer intends to blackmail Torvald.

Torvald can hardly believe it.

TORVALD. Nora!—No, I must read it once again—. Yes, it is true! I am saved! Nora, I am saved!

Nora looks at Torvald in stunned disbelief and asks coldly, "And I?"

TORVALD. You too, of course; we are both saved, both you and I. ... My poor little Nora, I quite understand; you don't feel as if you could believe that I have forgiven you. But it is true, Nora, I swear it; I have forgiven you everything. I know that what you did, you did out of love for me. ... You must not think any more about the hard things I said in my first moment of consternation, when I thought everything was going to overwhelm me. I have forgiven you, Nora; I swear to you I have forgiven you.

Nora is wholly dismayed by Torvald's reaction, and she's underwhelmed by his understanding and forgiveness, which come too late to save their marriage.

In those few minutes of Torvald's ecstacy and self-centered relief, Nora decides to leave Torvald and their children. Within another few minutes, Nora closes the door to their home behind her.

Kristine has a very different relationship history than Nora. Nora married young and has become used to fitting in her expected societal role and having her needs fairly well met so long as she fulfills her duties. Kristine, on the other hand, was too poor to marry, and this separation from the institution has given her some perspective.

Over the course of the play, Nora explains the circumstances of her marriage in great detail, and this gives Kristine a unique view of the situation that neither Nora nor Torvald are capable of having. Kristine wants Nora to see the ways that Torvald wishes to fully control his wife and family, because Kristine doesn't believe that Nora would choose to stay if she could fully see it. It turns out she's right.

Kristine's actions come to no surprise. From the moment she enters the play, we realize that she is the only person who knows Nora on a deeper level. She also makes evident her awareness of the condition of the Helmer marriage: it is tantamount to a very well-staged play, where Nora plays the role of a doll inside a doll's house.

Kristine is transparent from the start. She never hides the fact that the dynamics of the Helmer household make little sense to her. She also shows a preoccupation with Nora's behavior. She acknowledges that Nora is immature, naive, and clueless about true life issues.

When Krogstad confesses that he left the incriminating letter in the Helmer's letterbox, he wants to retrieve it, but is discouraged by Kristine on grounds that this letter will unveil the sham of the marriage, and resolve the lies, for once and for all. She asserts that perhaps, this "truth" is what the marriage needs to shake it off its foundations until only reality remains, that is, if there is any reality left in it.

[...] twenty-four hours have elapsed ... I have witnessed incredible things in this house. Helmer must know all about it. This unhappy secret must be disclosed; they must have a complete understanding between them, which is impossible with all this concealment and falsehood going on.

The falsehood, concealment and secrecy, in Kristine's mind, have served as pillars to sustain a house where her friend exists merely as an entertainment and mothering figure. Kristine has learned enough about life and strength to know that Nora can do better than acting like a doll to earn her husband's attention and gain a sense of self-worth. The letter issue is a form of "tough love" for her friend.

On the face of it, it seems like Mrs. Linde is betraying Nora. However, in actual fact Kristine's simply showing her friend some much-needed tough love. Although she knows that exposing her marriage as a total sham will cause serious problems between her and Torvald, she also knows that it will ultimately be of great benefit to Nora in the long run. Kristine's a fundamentally decent person who believes that honesty is always the best policy, so it's not surprising that she wants the truth to come out. But it was the truth of Nora's deceit that Kristine originally wanted to see exposed. However, Krogstad's letter has the added advantage for her in that the shallowness of Torvald's feelings for Nora will also be revealed once and for all.

Kristine recognizes something that Nora is incapable of recognizing; the marriage between Nora and Torvald is a sham based on materialism and lies. Kristine encourages Nora, numerous times, to tell Torvald of the deception and blackmail, but Nora refuses. Kristine understands that the only way their marriage can ever be real and honest is for Torvald to find out about the loan and lies that Nora has been hiding for years. Krogstad is the natural person to reveal this information to Torvald. Kristine also feels that she needs to maintain her honesty and moral virtue and not sink to the level of Nora’s lies and manipulations. 

Act III of A Doll's House shows Mrs. Linde and Krogstad opening up about many realities taking place in the play. One of them is their own realities; they are both older than when they first met. They are also alone, and in need of someone to take care of their emotional needs.

The second issue is the mirror relationship between Nora and Helmer. It is a reflective relationship to that of Linde and Krogstad in that new dynamics are taking place, only inversely. Linde and Krogstad are two lonely souls finding out about each other's sadness and, as a result, getting together at last.

Conversely, Nora and Torvald are a seemingly-strong marriage which is about to endure having to learn the truth about one another and, as a result, they will be dissolved forever.

Mrs. Linde is aware of this; she knows about the lies and pretenses that Nora lives under. She believes in Nora as a better and stronger woman than to live her life as Torvald's play thing.

Hence, the main reason is that Mrs. Linde wants Nora to experience a life without lies and unneccesary pretension. She wants her and Helmer to open up to each other and be a real couple. The fake issues must end. In all, it is all for Nora's own benefit.

This is why, when Krogstand decides to recall his letter, her answer is:

MRS. LINDE:...twenty-four hours have elapsed since then, and in that time I have witnessed incredible things in this house. Helmer must know all about it. This unhappy secret must be disclosed; they must have a complete understanding between them, which is impossible with all this concealment and falsehood going on.