Lost Horizon is a 1933 novel by the English writer James Hilton. The book was adapted into a film, also called Lost Horizon, in 1937 by the director Frank Capra and a musical remake in 1973 by the producer Ross Hunter with music by Burt Bacharach. It is the origin of Shangri-La, a fictional utopian lamasery located high in the mountains of Tibet.

The novel has a frame story set in Berlin, where a neurologist obtains a manuscript which records the narrative of a British diplomat who had disappeared in China. The main narrative depicted in the manuscript starts in May 1931 within the British Raj. There is a revolution in the country and several people are evacuated. A number of them are transported in a Maharaja's airplane, but that plane is hijacked. After a crash landing, the four surviving passengers attempt to locate Shangri-La in the Kuen-Lun mountain range.

The prologue and epilogue are a frame story narrated by a neurologist. This neurologist and a novelist friend, Rutherford, are given dinner at Tempelhof, Berlin, by their old school-friend Wyland, a secretary at the British embassy. A chance remark by a passing airman brings up the topic of Hugh Conway, a British consul in Afghanistan, who disappeared under odd circumstances. Later in the evening, Rutherford reveals to the neurologist that, after the disappearance, he discovered Conway in a French mission hospital in Chung-Kiang (probably Chongqing), China, suffering from amnesia. Conway recovered his memory, told Rutherford his story (which Rutherford recorded in a manuscript), and then slipped away again.

Rutherford gives the neurologist his manuscript, which becomes the heart of the novel.

In May 1931, during the British Raj in India, the 80 White residents of Baskul are being evacuated to Peshawar due to revolution. In the aeroplane of the Maharajah of Chandrapur are: Conway, the British consul, aged 37; Charles Mallinson, his young vice-consul; an American, Henry D. Barnard; and a British missionary, Miss Roberta Brinklow. The plane is hijacked and flown instead over the mountains to Tibet. After a crash landing, the pilot dies, but not before telling the four (in Chinese, which only Conway speaks) to seek shelter at the nearby lamasery of Shangri-La. The location is unclear, but Conway believes the plane has "progressed far beyond the western range of the Himalayas" towards the lesser known heights of the Kuen-Lun mountain range.

The four are taken there by a party directed by Chang, a postulant at the lamasery who speaks English. The lamasery has modern conveniences, like central heating, bathtubs from Akron, Ohio, a large library, a grand piano, a harpsichord, and food from the fertile valley below. Towering above is Karakal, literally translated as "Blue Moon," a mountain more than 28,000 feet high. Mallinson is keen to hire porters and leave, but Chang politely puts him off. The others eventually decide they are content to stay: Miss Brinklow because she wants to teach the people a sense of sin; Barnard because he is really Chalmers Bryant (wanted by the police for stock fraud) and because he is keen to develop the gold mines in the valley; and Conway because the contemplative scholarly life suits him.

A seemingly young Manchu woman, Lo-Tsen, is another postulant at the lamasery. She does not speak English, but plays the harpsichord. Mallinson falls in love with her, as does Conway, though more languidly. Conway is given an audience with the High Lama, an unheard-of honor. He learns that the lamasery was constructed in its present form by a Catholic monk named Perrault from Luxembourg, in the early eighteenth century. The lamasery has since then been joined by others who have found their way into the valley. Once they have done so, their aging slows; if they then leave the valley, they age quickly and die. Conway guesses correctly that the High Lama is Perrault, now 250 years old.

In a later audience, the High Lama reveals that he is finally dying, and that he wants Conway to lead the lamasery. The High Lama then dies. Conway contemplates the events.