Jessica has run away from her late husband's family. Found again by her handsome and domineering brother-in-law Lord Raeburn, she returns with him to the family estate with her baby. Facing intrigue and suspicion, she learns slowly that perhaps she married the wrong brother, although now Lord Raeburn is engaged to someone else.
Severity reflects intensity, not value — “central theme” means a warning is a core part of the book, not that the book is bad.