Rachel Verinder is supposed to inherit her uncle Herncastle's Indian diamond on her eighteenth birthday. Of course, her mother has never trusted her brother, and is worried that he might have left Rachel the diamond as a way of getting his revenge on her. Franklin Blake and Gabriel Betteredge are anxious for the same reason, especially when three Indian jugglers, or street entertainers, show up at the house on the same day that Franklin Blake arrives carrying the Moonstone.
The obvious explanation is that the Indians must have stolen it, since they were nearby and because they had the motivation (after all, John Herncastle had stolen the diamond from a temple in India years and years before). But the Indians have an airtight alibi, so it must have been someone in the house. But who could it be?
Rachel refuses to cooperate with the investigation and leaves the house. She stops talking to Franklin Blake and is incredibly rude to him, even though they'd been close to getting engaged before her birthday. Meanwhile, Rosanna Spearman, one of the servants, has been acting strangely. She is secretly in love with Franklin Blake, and might know something about the missing diamond. Unfortunately, she commits suicide before confessing anything.
Franklin Blake finally reads Rosanna's suicide note and discovers that he must have been the one who took the diamond from Rachel's sitting room. But he has no memory of doing so! Is he going crazy? Was he walking in his sleep? How on earth could this have happened?
Franklin assumes that he must have been framed. He doesn't remember taking the diamond, and it's possible that someone else was wearing his nightgown when they went into Rachel's room to take it. So Franklin confronts Rachel for an explanation. Why does she suspect him? Did Rosanna show her the stained nightgown? No – Rachel tells him that she saw him take the diamond with her own eyes. So Franklin is right back where he was: he must be going crazy.
Franklin Blake happens to meet Ezra Jennings, the doctor's assistant. Jennings hears the story, and figures out that Franklin must have been drugged with opium the night that the diamond disappeared. He could have been in an opium-induced trance when he took it, which is why he doesn't remember anything. They conduct an experiment to see whether this theory holds water, and Franklin Blake's innocence is proven! Unfortunately, they still don't know who took the diamond and pawned it in London…
Godfrey Ablewhite is found, murdered, after having picked up the diamond at the bank where he'd pawned it. The three Indians have killed him and taken the diamond back to India. Rachel and Franklin get married.