This is the story entitled "A Doll's House".
Mrs. Linde wrote Krogstad a letter long ago to explain her decision.
Mrs. Linde and Krogstad were lovers when they were young. However, during those time Mrs. Linde had no choice but to marry another man. A man who has money and can help her fend for her family. She left Krogstad a letter telling her decision to leave him and marry another.