Oswyn takes Christopher through hallways and offices until they reach an empty one, then scolds him for accusing a master. He does not believe Christopher until Christopher explains that Stubb's apprentice is the murderer. However, Oswyn says that Wat is not Stubb's apprentice; Stubb has two, both of whom Oswyn knows. He questions why Christopher thinks this, and Christopher explains that he was in the shop. He tells Oswyn what he heard, including that the men were looking for fire, but this confuses Oswyn. 

Oswyn reveals that Master Benedict's will has gone missing from the vault. Without a will, the shop goes to the Guild, and Stubbs can buy it. Oswyn confirms that, contrary to what Master Benedict said, the Cult exists. Christopher wonders what they want, and Oswyn tells him that they want power, which they hope the fire will unlock. Christopher realizes that they want to use it to get rid of the king. 

Oswyn tells Christopher that he needs someone to verify his account, but Christopher lies and says that he was alone. Oswyn believes this will be a problem, as Sir Edward will not want to believe that a Guild member is associated with the Cult. Furthermore, Christopher has already undermined his authority being disrespectful to Stubb in front of the Guild members. Oswyn promises to send someone to go through the shop and speak to Sir Edward himself. He tells Christopher he can stay in the office until he can speak to the Grand Master. On his way out, Oswyn warns Christopher about lying, and Christopher—despite having stated that he was alone—swears that he has told the truth.

As Oswyn leaves the Guild Hall, encountering a man in the courtyard, Christopher watches him. Once Oswyn has left, Christopher realizes that Stubb might show up and leaves. He returns to the Baileys' home. He asks Tom's mother if he and Tom go with him to a church service. However, they go back to Hugh's house instead. 

Determined to find clues, Christopher shows Tom that Hugh also uses a brick marked with the symbol for Mars to hide the extra key to his house. Tom worries that Hugh will discover them, but Christopher is relatively sure that Hugh is in the country. If he had been murdered, Christopher rationalizes, they would know because it would have been conspicuous. Inside the house, they find that burglar has been there. The workshop has not been used in a long time. 

Searching for "Hugh's fourth" (189), Christopher still isn't sure what this means but hopes to find out. They comb through the house and start looking through Hugh's papers. As they read, Christopher gets uncomfortable and takes off the apothecary sash. Tom begins to look through it. Meanwhile, Christopher reflects on the meaning of "end.swords" (192), uncertain what the punctuation means. When Tom pulls out oil of vitriol, Christopher warns him that it dissolves anything. They go to try it on something so Christopher can see. 

At the window, they find that Bridget has followed them. Looking out, Christopher realizes what Hugh's fourth means. He points out to Tom that they are on the fourth floor. Furthermore, the view shows them a private garden that has a gate with two iron lion statues on it.

Christopher and Tom try to find the private garden but have a difficult time because of the walls between houses, which confuse them. Eventually, they decide to follow Bridget. Reviewing the message, Christopher interprets the message to mean that they should go through the gate beneath the lions. There is a small mausoleum in the garden, which has gone to ruin. Exploring the outside of the house, Christopher sees that the building is dedicated to a man with the last name "Mortimer," like one of the men the Cult killed. Tom realizes that from a certain point of view, a distant statue of St. Peter appears to be holding a key in the direction of the mausoleum.

Inside the mausoleum, the boys find a sarcophagus. It is inscribed with Latin words meaning "The Lord is my light." There are three alcoves, each of which contains a statue. One shows a man with a tower and a book. Another shows a man with a lion's paw at his feet. Christopher recognizes these as the saints Thomas Aquinas and Jerome. The third statue is that of an angel with a sword. This reminds Christopher of the phrase "end swords," and he follows the sword's path to a stain on the sarcophagus that looks like the symbol for Mars. He presses it and the sarcophagus moves to reveal a hole.

Christopher wants to go down into the hole, but Tom hesitates. Christopher lights a torch and goes down anyway; Tom follows. Beneath them is an old crypt lined with skeletons. They go down a passageway to a workroom. There, they find numerous jars with substances apothecaries use. They also see an enormous mural of an angel killing a dragon, with two more dragons standing by and a snake framing the picture. Based on the angel in the mural, Tom believes this is the headquarters of the Cult. However, Christopher knows that his master wouldn't have sent him there.

The boys start evaluating the room's purpose. Christopher thinks it could be a storeroom. Tom worries that they're being watched, pointing out that there are holes in the dragons' mouths on the mural. Feeling one, Christopher realizes the wall is made of iron. He also notices that there are symbols in the painting by the dragons, including of one of a divided triangle, a more complex series of symbols, and the symbol for Mercury. Looking at his puzzle cube, Christopher realizes that the wall is actually a door.

Christopher explains his discovery to Tom, guessing that pouring quicksilver into the third hole will open the door. Christopher puts quicksilver in and hears a noise, but nothing happens. He realizes they need to understand the other symbols. Tom wonders if the coded message contains the answer, but Christopher isn't sure. 

The boys leave the sarcophagus, as Christopher has to go back to the Hall. Christopher leaves his puzzle cube at Tom's house. There, Tom's sister Cecily tells Christopher to run. Before he can, Tom's father grabs him and throws him out into the street. As Tom goes to help Christopher, his father hits him. The constable came by the house for Christopher, explaining that Lord Ashcombe wants him. Thinking Christopher is in trouble, Mr. Bailey wants Christopher to leave and not come back. Mrs. Bailey apologizes but agrees.