The premise is that a group of men are held as hostages by the Germans in Occupied France, and a decimation order is issued -- one in ten are to be executed in reprisal for Resistance action. The plot bears similiarities to Doctor Fischer of Geneva. Part of it also seems like an inversion of the story of St. Maximalian Kolbe, a priest who was executed in Auschwitz after volunteering to take the place of another man following just such a decimation order. Here's his story: http://www.catholic-pages.com/saints/st
If nothing else, people who enjoyed Our Man in Havana will want to read "Nobody to Blame," one of two sketches Greene includes in the Introduction. This (and the other, "Jim Braddon and the War Criminal") was originally going to be a film script, but instead it grew into Greene's novel.