Warning: this annotation reveals the solution of the novel's mystery. The Moonstone is typically read as one of the first detective novels in British fiction. Published serially in 1868, the novel sets out to identify the thief who steals an expensive gem, the moonstone, from the heroine, Rachel Verinder. Franklin Blake, Rachel's love interest, turns out to be the thief. He stole the gem while under the influence of a dose of opium slipped to him by the local doctor. He has no memory of the deed.

Blake is able to solve the mystery--and win Rachel--with the help of Ezra Jennings. Jennings is also a doctor, but his unorthodox appearance and methods make him a pariah. Jennings is addicted to opium, which he began prescribing for himself to reduce the pain of a chronic illness. He uses his knowledge of the drug to re-create Blake's state on the night the jewel disappeared. Blake again tries to take the jewel and the mystery is unraveled.