After Eliza Scarlet's father dies, she takes over his detective agency, against the wishes of everyone around her, including Inspector Wellington (aka the Duke). Can she prove to herself that she has got what it takes to be a detective?

As London's only female detective, Eliza is struggling to get work. Desperate for money, she takes a seemingly impossible case; proving the innocence of a man found at a murder scene with a bloodied knife in his hand who has pleaded guilty. Along the way Eliza clashes with Duke, who sees this as an open and shut case. But when she solves the case, Duke grudgingly begins to take her seriously.

Scotland Yard have their eye on many potentially disruptive gangs and political groups. Eliza is hired by Duke to go undercover and infiltrate one such group. Eliza is delighted, this is progress and a show of faith in her abilities. She is less delighted when she learns who she will be spying on -- a women's suffrage group campaigning for the vote.

There was a macabre Victorian custom where people had a ‘Death Photographer’ capture the image of a deceased relative. When one such photographer receives threatening messages from beyond the grave, Eliza must also reflect on her own late father.

After finding her father's lost casebook, Eliza looks more closely at how he died. The final entry in Henry Scarlet's casebook takes Eliza to an abandoned prison on the outskirts of London. She and Duke investigate what seems like an empty building, but they soon discover that it is the hideout for a criminal gang.

Eliza is determined to discover who murdered her father and why. Moses is the prime suspect, but the real murderer may be someone who has been under Duke's nose the entire time.