Alexander Blake, a handsome former soldier and respected detective inspector joins the force at Scotland Yard to replace the Duke who has gone to America. Shortly after he starts his new job, he meets private detective Miss Eliza Scarlet. He's not particularly shocked by a woman working as a private eye, so Eliza takes this to mean she'll be given more cases. But their relationship gets off to a rocky start since Blake has decided not to allow private detectives to aid in his investigations. As Blake and Miss Scarlet cross paths at various crime scenes across London, they can't help but develop mutual respect for one another, and perhaps even an attraction.

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. 

Eliza Scarlet is immersed in solving new mysteries in Victorian London as growing competition from a rival detective agency creates new problems. Has someone new caught the eye of Inspector William Wellington, the Duke?