Holmes and Watson set out to bring down a blackmailer, but someone else gets to him first. In the process, Holmes gets engaged to a woman who works in the blackmailer's household, simply to get information. This is stated only in one line to explain how Holmes got the information. We never see the woman, and it is never mentioned again in the story. There are a hundred different ways to explain that away, taking advantage of a random woman wasn't necessary. It's especially ironic that the blackmailer was brought down by a scorned woman, but we never hear anything about what Sherlock did to this one.