[Chapter 40] What startling revelation does Huck come to regarding Jim?

Huck discovers that Jim was always a free man. Mrs, Watson had released him.

Huck realizes that Jim is "white inside." Tom has been shot, and instead of escaping Jim stays to make sure that Tom is alright. Huck needs to get the doctor; Jim says that if their roles were reversed Tom wouldn't leave him. Therefore, he is staying.