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Perdita, the widowed Duchess of Ormand (never called the dowager, perhaps because her husband's grandmother is still alive?), became the duchess one night when her husband was trying to kill her and he was killed instead.
Her husband's secretary, Lord Archer Lisle (fifth son of a duke), wasn't in the room when death occurred but he learned of the circumstances soon after. He is ashamed that he didn't realize sooner what the duke was doing to Perdita, especially because he loves her. But, as the fifth son of a duke, he has hopes that he can marry Perdita but is realistic about how society will view the marriage. Plus, there's the inconvenient fact that Perdita will not marry for love (look where that got her the first time!)
Of course, the two are forced together, working to discover who blames Perdita for the duke's death and why they're trying to thoroughly discredit her before they kill her.
Previous books: Why Dukes say I Do, Why Earls Fall in Love