Franklin picks up where Jennings's diary leaves off. Back in London, Franklin and Mr. Bruff are met by Mr. Bruff's small spy, Gooseberry, who informs them that Mr. Luker has left his house for the bank. Franklin sends the women home and goes to the bank with Bruff. At the bank they are informed by more of Bruff's men that Luker has entered the inner office of the bank and has not yet come out. They see no Indians around, but there is a tall, dark- complected man with a wide face and full beard who looks like a sailor.
When Mr. Luker emerges, they think they see him hand something to a man in a gray suit. Bruff's men follow Luker, and Bruff and Franklin follow the man in the gray suit, who turns out to be a chemist who knows nothing of the Moonstone. Gooseberry disappeared in pursuit of someone else, and Bruff and Franklin wait for him at Bruff's office. Franklin eventually returns home to find that Gooseberry has called for him there and will return in the morning.
The next morning, Franklin receives a visit from Sergeant Cuff. Franklin fills him in on the updates of the case. Cuff gives Franklin an envelope with the name of the person Cuff suspects as the thief inside and tells Franklin not to open it until he finds out the truth. Gooseberry soon arrives and tells Franklin that he followed the dark-complected sailor from the bank. Franklin is called from the room, and Cuff continues questioning Gooseberry.
When Franklin returns, Cuff and Gooseberry have prepared to take a cab to the east of the city. In the cab, Cuff explains that Gooseberry followed the sailor to a steamboat and then to an eating-house. Gooseberry had noticed that the man was also followed by a man dressed like a mechanic who was working for the Indians. Gooseberry followed the mechanic who followed the sailor to a pub and inn called "The Wheel of Fortune." The sailor let room number 10 for the night. After a little while Gooseberry noticed a disturbance. The mechanic had gone to room 10, drunkenly claiming it was his room and was thrown out of the pub. Gooseberry had followed him to the street and saw that he really wasn't drunk. Gooseberry, confused, had returned to Franklin's house to report. Cuff guesses that the mechanic pretended drunkenness to force his way in and examine the layout of the room the sailor was to sleep in with the diamond and report back to the Indians.
They reach "The Wheel of Fortune" in the cab and find that the sailor has not budged from room 10 all morning and will not answer the landlord's knocking. The landlord gets a carpenter to take off the door. The sailor is on the bed, dead. Gooseberry finds the box and receipt for the Moonstone. Cuff shows Franklin that the sailor's face is a disguise of makeup. Pulling off the beard and washing off the makeup, Cuff reveals that the sailor is Godfrey Ablewhite—the same name of the culprit that Cuff has given Franklin in the envelope.
Chapter I of Cuff's narrative is a letter to Franklin dated July 30, 1849. Cuff writes to say that his report of the investigation is not quite complete and will be given to him soon.
Chapter II of Cuff's narrative is a report citing clues and asserting that Godfrey was smothered by a pillow by the Indians who wished to gain the Moonstone. The Indians entered the room through a trap door. A piece of gold thread, manufactured in India, was found in the room. The Indians were traced to a steamboat that left London on the morning of June 27 bound for Rotterdam.
Chapter III continues the report, stating that Godfrey led a double life: that of a philanthropist and that of a "man of pleasure" under a different name. Cuff has discovered that Godfrey lived sumptuously on the trust fund of a minor for whom he was acting as trustee. The minor's eighteenth birthday was to occur in February of 1850—at that time, Godfrey would have to sign over the money he had already spent. Godfrey was thus in need of money on the night of Rachel's birthday and had first hoped to get it by marrying her, but she turned him down. Mr. Candy asked Godfrey to administer the laudanum to Franklin, and he agreed.
Chapter IV consists of the report of Mr. Luker's statement. Godfrey told Mr. Luker of how he, Godfrey, had noticed Franklin enter Rachel's room on the night of her birthday. When Franklin returned, he noticed Godfrey in the hall and called to him. He asked Godfrey to take charge of the diamond and put it in his father's bank. When, the next morning, Franklin clearly did not remember his actions from the night before, Godfrey took the diamond.
Chapter V reports that Luker consented to lend Godfrey money in exchange for the Moonstone. Godfrey could redeem it in a year's time with 3,000 pounds. A year later Godfrey was able to redeem the gem with the money left to him by an elderly charity woman. He had planned to take the gem to Amsterdam and have it cut up and sold. Instead, the diamond is with the Indians on a boat set to dock at Bombay. Cuff has alerted the Bombay authorities, who will board the ship as soon as it enters the harbor.
In the fifth narrative, told by Franklin Blake, Sergeant Cuff is reintroduced to the story—indeed, he has come out of retirement to resume the case. Sergeant Cuff's dismissal and retirement in the First Period of the novel was somewhat surprising: the beginning of the novel sets us up for a mysterious case to be solved not by the bumbling Seegrave, but by the famed Sergeant Cuff. However, Cuff is soon found to be on the wrong trail and is dismissed from the case by Lady Verinder, in an abrupt departure from the novel's implied plan. Thus it is interesting to note that The Moonstone, which paves the way for the English-language tradition of the detective novel, also features a fallible detective—as Cuff himself puts it, "It's only in books that the officers of the detective force are superior to the weakness of making a mistake." Collins's real-minded attitude toward the character of Cuff, as evidenced in this quotation, is consistent with his treatment of other characters as well—Franklin and Rachel, for example, are not afforded the idealistic, sentimental depictions of other heroes and heroines of Victorian novels.
Cuff reappears in these final chapters to see the case through to the close, and his correct suspicion of Godfrey Ablewhite is by this time unsurprising to us. Godfrey has been shown to be hypocritical and capable of leading a double existence in the narratives of Miss Clack and Mr. Bruff. Additionally, he is shown to be in search of money, thus supplying a motive. The setting and circumstances of Godfrey's murder suggest his duplicitous character. The name of the pub, "The Wheel of Fortune," is appropriate to a gentleman who has secretly run out of money, and the disguise that Godfrey wears are a symbolic representation of his own double life.
The triumph of the Indians in stealing the diamond, along with their murder of Godfrey, is presented within the sphere of poetic justice. Godfrey, as a casualty, is parallel to the three Brahmins killed by John Herncastle in India in 1799. Since Godfrey is discovered to be a dishonest and evil man, his death is not much mourned among the other characters. The search for the diamond continues, but, notably, no one has truly expressed a desire for the return of the diamond—there is an almost supernatural sense that the diamond brings ill luck with it when it is outside of India.
Cuff's final report to Franklin in which he explains the background of both his own detective work—and Godfrey's criminal past—has become a common characteristic of detective fiction. It is common for the detective, after he correctly guesses and exposes the criminal, to fill in the details of the crime, as well as the method of his detection.