The Duchess of Malfi is a widow, and her brother Ferdinand does not want her to remarry, so that he will eventually inherit her estate. So, he sets a spy on her, Bosola.
Despite Bosola’s efforts, the Duchess marries her steward, Antonio. It’s not clear what would have happened if she had picked someone closer to her station, but this choice outrages her brothers. (Oddly enough, Bosola doesn’t report that she has a lover until she has three children by him.)
At first, the brothers think the Duchess has been whoring around, but the situation isn’t improved by their finding out she is actually married. Ferdinand has her imprisoned in rooms of her castle, and things get worse from there.
When I studied 17th century drama, these plays were called revenge tragedies, but the introduction to my very old Mermaid edition calls them Tragedies in Blood. Since pretty much all the main characters are dead by the end, this is a fitting name.
Webster’s play is a bit rough around the edges. Certainly, it doesn’t have the power of Shakespeare or even Marlowe, and most of it is in prose. Still, there are some effective moments. I think this play is probably much more moving when performed rather than read. I read this play for my Classics Club list.