The novel Flashman and the Mountain of Light is one of a series of novels posing as the memoirs of Sir Harry Flashman. This novel, the "ninth packet" of the Flashman Papers, chronicles Flashman's adventures in the First Sikh War. Through an unbelievable series of events Flashman, acting as a spy in the Punjabi capital, meets a series of unforgettable characters, some who equal his own capacities for debauchery and cowardice.

The novel begins in 1887 when Flashman and his wife Elspeth have an audience with Queen Victoria. The Queen asks Flashman's advice concerning the Koh-i-Noor, the largest diamond in the world at that time, and the upcoming Golden Jubilee celebrations. Hearing of the diamond takes Flashman back in time over forty years to the First Sikh War.

Flashman arrives in India and soon acts to get assigned as a political service officer because he thinks the job will be easier and far less dangerous than his usual role of soldier. Flashman soon discovers that he has volunteered for what may be the most dangerous job of any British man in all of India. Under the direction of George Broadfoot, Flashman goes to Lahore where the ruling family of Punjab holds court. He is supposed to act as a spy relaying vital information to the British concerning what most believe to be an inevitable war between the British East India Company and the Punjabi military, the formidable Khalsa. Whether or not Flashman is an effective spy is uncertain, but what is absolutely certain is that he encounters an environment where his appetites for excess are tame by comparison.

The first memorable character Flashman meets during his assignment is his assistant Jassa. Jassa is not at all what he initially seems. In fact, his true identity and history shocks even Flashman, who dislikes the character at first. We later discover that the dislike might be based on the two men's many similarities.

At the court of Lahore where Flashman is supposed to be gathering intelligence under the guise of attempting to settle an inheritance dispute Flashman divides his time between two beautiful women. Maharani Jeendan is the seductive acting ruler of Punjab who spends her time drinking alcoholic beverage and collecting lovers. Mangla is Jeendan's trusted servant who puts physical demands on Flashman after Jeendan is temporarily satisfied.

When Flashman is not being drained of all energy and will by the appetites of the two women, he finds himself in one life-threatening situation after another. Once he escapes being roasted alive by the Khalsa only to find that he has to voluntarily join them again. Each time he escapes one situation he finds himself in an even more dangerous one, and they are usually devised by people who believe in Flashman's undeserved reputation as a hero. These people believe in Flashman "the Afghan slayer" when Harry Flashman himself unashamedly tells us that he is in fact the worlds greatest coward.

At the center of all this adventure is the world's largest and most valuable gem, The Mountain of Light.