In the new season, Eliza's agency is thriving and her professional life is on a successful path. On the personal side, however, Duke has now been away in New York for several months and Eliza is forced to make a difficult decision. Meanwhile, a new Detective Inspector comes to Scotland Yard and Eliza must navigate building a relationship with him if she's going to continue having access to police resources. All these changes and decisions beg the question: Can Eliza Scarlet have it all?

In Miss Scarlet and The Duke Season 4, Eliza has taken over the business of Nash & Sons (not that he has any sons) and things are not going entirely smoothly, although help comes from some familiar sources. Outside of work, her relationship with William "The Duke" Wellington builds towards a looming decision that will shape both their lives.