I disagree a little with another reviewer who said this Regency was predictable. I actually found it a little different than the rest. There were some flaws that would normally make me give it less than a 5, but because of the emotional charge I felt while reading this, I couldn't give it less!
Lady Isabelle Milborne survived an abusive marriage. During that marriage her husband made her a partner in a scheme to revenge himself on Justin Everard, the Marquis of Wroth. Belle was really an innocent in the matter of framing Justin and forcing a duel, but Justin believed she betrayed him. He was in love with her and felt his actions (winning her in a card game!) were justified to save her. Belle saw it differently, but to protect him, she pretended to have lured him into her husband's clutches. The duel ends badly and though Lucian Milborne lives (but does not recover completely), Justin flees the country and enters the military.
Three years later, Justin returns a hero and is now the Duke of Westmore. Belle is a widow, with a small pension and modest demeanor. But Justin is not fooled and sets out complete his own revenge. By implying he will court her young sister-in-law (Chloe) whom Belle loves as a sister, Justin forces Belle to become his mistress. Just the hint of scandal makes the ton snub Belle and she is no longer even allowed to talk to Chloe! Justin has a conscience, and removes Belle from London to his seaside home. He shows himself to be a wonderful gentleman who continues to love and care for Belle. Though they both try to remain impersonal and remote, they cannot hide the growing attraction and love.
There are villains that force separation of the lovers. All the side characters are well fleshed out and I see a story or two coming out of them. You cannot see how this will end. Belle with her protective qualities always plays second fiddle and Justin, too, places duty above love. Nice reading - and of course the happy ending is achieved.