The central theme of The Magic Mountain is time: what duration, habit, and illness do to a person removed from ordinary life. Around it, the novel deals with death, with the formation of a young man without direction, and with the clash of ideas that contended for Europe before the Great War.
Hans Castorp goes up to a sanatorium in Davos for a three-week visit and stays seven years. The narrator warns at the outset that he will tell the story with exactness and thoroughness, and the length is deliberate: duration is the book's own instrument, and a novel about what duration does to a person could not be short. The thesis appears in the Excursus on the meaning of time, in Chapter IV: habit weakens the sense of time, and boredom is a "morbid brevity" produced by monotony.
The other themes pass through the same axis. Illness keeps those who go up healthy; death goes from the grandfather's burial, in childhood, to the death of cousin Joachim; and the two pedagogues, Settembrini and Naphta, contend for the protagonist's mind without the novel giving either one reason. Beneath everything is a question that Chapter II formulates: Hans Castorp's era does not know how to answer "What for?", and a young man without that answer is open to any system that is offered to him.