Despite being criticized and belittled by the superintendent for a report on crime, William is ordered to attend drinks at a club that night so that he can offer his “little facts and figures” to other officers on behalf of the superintendent. Just as William finished donning his formal dress, Ivy interrupts him: Eliza has disappeared.
Ivy had been away for a day; when she returned in the morning, Eliza wasn’t there. Her bed hadn’t been slept in, her office was locked up, and she atypically had not informed Ivy of her whereabouts. She last saw her the previous morning, going to a meeting with Rupert.
William sets about reconstructing Eliza’s movements, trying to locate her before his meeting at the club. He finds Rupert with Herr Hildegard and his niece. Eliza was agitated, having made some discovery, so she postponed the meeting with Rupert. Herr Hildegard then helped her get a cab to the morgue.
While there, she questioned the mortician about smallpox, he tells William. That ends the trail, so William visits Eliza’s office (he has a spare key). He finds a casebook with a page ripped out, and shades the page beneath to decode the pen marks: “Woolwich Prison Cell 99.”
At the abandoned prison, William finds the boards over an entrance broken. He spots Eliza’s hat on the ground, and hears a faint yell for help in response to his calls. Eliza is locked inside cell 98. She went inside and then the door slammed shut behind her—she may have heard footsteps. Her bag, with its lock picking tools, is outside the door.
As William tries to pick the lock, the explains that the prison and cell 99 were mentioned in her father’s casebook several times—including on the last page. But the cell numbers only go up to 98. William and Eliza’s conversation moves through their typical compliment-insult-argument trajectory, leading Eliza to tell William to hand her the lock picking tools and leave her be.
While she works on the lock from the inside, William wanders away and finds a room full of keys—and a man with a knife. William draws his gun, but the man manages to swat it away and the confrontation turns into a fistfight. Eliza finally gets the cell door open—and up walks William, holding the keys.
His arm is wounded, so Eliza cuts off a piece of his expensive coat to stanch the blood. The other man was also wounded and ran off. William’s pocket watch, a gift from Henry, is broken. As Eliza and William leave the cell, he stumbles and drops his gun over a railing.
The wounded man reappears and grabs William’s gun to shoot at them. They take shelter in a cell, and the man collapses. While William goes to attack him, Eliza distracts him, and William knocks him out. They tie him up and find a key to cell 99 on his person. Plus, Eliza recognizes him: he’s the “doctor” who found Henry dead in the street and brought his body home. Henry must have been investigating something serious, and was murdered for his prying.
Eliza and William are still locked in the prison—the man locked the gate out. As they try various keys, he rouses and says he called for reinforcements. A burly man holding a crowbar begins descending the stairs. William’s gun is out of bullets.
Finally, Eliza gets the gate open. As the man charges at William, they slip through the gate and lock it behind them. As they search for an exit, Eliza explains that the prison was closed a couple years ago due to a smallpox outbreak. When they come upon a room full of gas masks, she notes that they were used for fumigation, so that rats wouldn’t take over the deserted prison.
When they finally find the exit, they hear a voice calling out: “Help!” Going down a new corridor, they find cell 99. A man cower inside when they open the door. Both the cell and the hallway are filled with counterfeit bills—the man, whose name is Nathaniel, says the people who locked him up forced him to make them. He’s been there for some three months, and had given up forgery before they threatened him with violence.
Shots ring out elsewhere in the prison. Nathaniel makes a run for it, out of the prison—and is shot by a man in a gas mask. The man turns the gun on William, who had chased Nathaniel, until Eliza appears, pointing William’s (empty) gun at the man. The man backs way and flees.
While William goes to get help, Eliza puts pressure on Nathaniel’s wound. He recognizes her as Henry’s daughter, then passes out.
The counterfeit bills amount to tens of thousands of pounds. The two thugs who attacked William and Eliza in another part of the prison have been shot dead. The gang behind this scheme is clearly dangerous—and they know where Eliza lives, having brought Henry’s body there. William will assign her police protection, and insists that she leave the investigation to him for her own safety.
William briefs the superintendent on the case—and finally acknowledges Eliza’s role in it, as well as her help with previous cases, after the superintendent disparages her and Henry.
The forger, Nathaniel, having been rushed to the hospital, will live. He kept saying a man was coming for him: a Jamaican named Moses.
At Eliza’s house, Moses sneaks past police protection to meet her. She asks him to get her information on Nathaniel.
At the hospital, Nathaniel is smothered in his bed.