The novel has three types of characters: those who are purely fictional; those based on real people but fictionalized; and those who were actual figures in the war.
Set in the Sierra de Guadarrama mountain range between Madrid and Segovia, the action takes place during four days and three nights.
The title of the book is a reference to John Donne's series of meditations and prayers on health, pain, and sickness (written while Donne was convalescing from a nearly fatal illness) that were published as a book in 1624 under the title Devotions upon Emergent Occasions, specifically Meditation XVII:
"No man is an Island, intire of it selfe; every man is a peece of the Continent, a part of the maine; if a Clod bee washed away by the Sea, Europe is the lesse, as well as if a Promontorie were, as well as if a Mannor of thy friends or of thine owne were; any mans death diminishes me, because I am involved in Mankinde; And therefore never send to know for whom the bell tolls; It tolls for thee."