In the books, Leithen is a Scottish barrister and a Conservative politician (and MP) and, at one point, Attorney General.
He served in the British Army during World War I, starting as a private in the Grenadier Guards and ending as a GSO 1 (General Staff Officer - Grade 1, probably a lieutenant-colonel or colonel).
In a number of ways, Leithen resembles Buchan himself, moving amongst high society, and espousing Conservative values.
He decides to spend his last days tracking down a missing financier who has had a nervous breakdown and fled to Canada.
Leithen finds the man but decides to stay with some Indians and help them fight a disease epidemic.