The novel is a day in the life of Henry: he gets up, drives to the gym, and comes home. Henry's day, however, is not entirely plain. Saturday takes place during London's largest Iraqi war protest, and in order to get to the gym, Henry must maneuver around it. As he takes a detour, he gets in an accident with another car that happens to be driven by several hooligans. The bizarre encounter winds up serving as the catalyst for the rest of the plot.