The Duchess of Malfi was one of the texts I read for AS English Literature, and I loved it.
The plot is slightly crazy; with The Duchess' husband having just died and her brothers the Cardinal and Ferdinand both petitioning her to re-marry a certain type of man. However, the Duchess is in love with Antonio, one her servants. Ferdinand, the Duchess' twin (who has some *complicated* feelings towards his sister) hires Bosola to spy on the Duchess; and he discovers that not only has the Duchess secretly married Antonio and is pregnant by him.
As it is a Jacobean tragedy, all manner of crazy things happen, and invariably most of the leading characters end up dead, or crazy.
I loved the Duchess as a character; it was such a surprise to read such a strong female character created by Webster during the Jacobean era. She is maybe one of my favourite characters that I have come across in English Lit classes.