In the wake of his father's death, Franklin Blake returns home and starts to look into the theft of the Moonstone, which was stolen on his cousin's birthday a year ago. Franklin needs the family butler to walk him through the events of that night.
Sergeant Cuff catches Franklin up on how his investigations went. Franklin is discomfited to learn of the suspicious activity of Rachel and how her maid might be implicated in the theft.
As Franklin continues to look for the truth, Rachel's cousin confirms that Rachel knew the identity of the thief. She relays to him how Godfrey Ablewhite and a jewel broker were kidnapped by Indian men, an event that appears related to the theft.
Franklin's solicitor helps Rachel find a way to break off her engagement to Godfrey. Franklin makes a shocking discovery about the identity of the thief who took the Moonstone.
In an attempt to confirm what Rachel told him, Franklin tries to recreate the events of the night the Moonstone was stolen, leading him to learn of the true nature of the sleeping draught he was given. A shocking death turns everything upside down.