Kitty drops off the turtle, Clyde, at Watson's. She is glad Kitty is making friends outside of Sherlock. Both Kitty and Watson receive a text from Sherlock.
Kitty goes to the 39th Street Library where some maps were stolen. They believe it was an inside job since a security guard has not shown up for work. Kitty observes there are scratches outside one of the locks, but the others are clean, suggesting they couldn't pick the locks and needed keys. She finds the security guard dead.
Kitty explains to Gregson she knew the body had to be in the room because of the other guards in the building. The killer couldn't move him from the room. Sherlock believes the thief was after one map, a King James County Map donated by the Brays. Sherlock believes the thief has done this before and convinces Margaret Bray to discreetly reach out to others.
Sherlock identifies the thief, Stuart Zupko. The police go to arrest him and find him dead.
The Bray map is on display on a table, but Sherlock says it is a fake.
Kitty and Gregson explain to Margaret the map is a fake. They are not sure if her Grandfather knew that when he donated it to the library, but the thief may think she has the original copy.
Sherlock learns the Bray map is part of a larger atlas, Smythe's Virginia. Sherlock and Joan go to the law firm that handled its sale. The buyer was William Hull, a real estate magnate. He wants to hire Sherlock and Joan to find the Bray Map. Sherlock walks out of the meeting. He tells Joan that the value of the map isn't the map itself, but the information on it.
Sherlock, Joan and Kitty examine the fake Bray map and compare it to photos of the real Bray map to see what's missing. Kitty's phone rings. Sherlock commands her to not answer it. Watson says to answer it. Kitty says she has to go to the loo. Watson tells Sherlock to butt out of Kitty's love life. Sherlock says Kitty had been badly damaged by what was done to her and he is trying to protect her.
Sherlock realizes they shouldn't have been looking for discrepancies between the real and fake map, but differences between the fake map and reality. The forgery is very close but the river has shifted a bit. There is an Indian casino slated to be built there but because of the shift in the river, the land is technically not on the reservation.
They go to a sister casino and the pit boss admits they had a forged map created and planned to pay someone at the library to swap the maps, but didn't get a chance. Watson looks at the model for the new casino. She knows who the killer is.
Margaret Bary comes to the precinct. Watson mentions she remembered that the Bray's owned a lot of land in Virginia. In fact, they stood to make quite a bit of money if the Indian Casino was built. She had Zupko steal the map, then had a forged one made and killed Zupko. Unfortunately for her, the forger actually kept the original and gave her another forged copy.
Sherlock apologizes to Kitty for trying to keep her away from Zachary. Kitty says she knew what he was doing and felt protected and loved. Sherlock suggests she invite Zachary to join them at the museum this afternoon.