Franklin Blake is tasked by his uncle to present the fabled Moonstone diamond to Rachel Verinder on her birthday. But the diamond is said to be cursed and three Indian conjurers are lurking nearby, intend on returning the jewel to it's place of origin.

The Moonstone has been stolen from Rachel's cabinet. Famous police Sergeant Cuff takes on the investigation. Even though second maid Rosanna Spearman is acting very suspicious, the inspector suspects the case to be more complicated.

Limping Lucy has a letter from Rosanna Spearman to be delivered personally to Franklin Blake. But unfortunately Blake has left the country. Godfrey Ablewhite is assaulted and searched by the three Indians in London. Afterwards he proposes marriage to Rachel once more.

Rosanna's letter offers explanation but also more mystery concerning the theft of the Moonstone. Franklin Blake sets out to prove his innocence and win back the love of Rachel Verinder. Doctor Candy's assistant Ezra Jennings offers his help.

Ezra Jennings conducts an unusual experiment to prove Franklin Blake's innocence. In order to do so, the exact conditions of the night the Moonstone was stolen have to be duplicated.