Please use the Question Tags to state the name and full version number of the Story Format you are using, as answers can vary based on this information.
When you say "import this html to Unity" do you mean:
1. How to display the contents of the Story HTML file within a your Unity based game using a custom web-browser plugin like UniWebView 3?
2. How to import and convert your Story HTML file to a programming language (like C#) that is supported by Unity, like the suggested Cradle plugin does.
3. Something else entirety...
warning: As explained in the Cradle plugin's documentation this plugin has selective / limited support for the features of the Harlowe & SugarCube story formats, and that support may require you to rewrite some parts of your project's functionality.