Comic book artist Clayton Riddell is celebrating a successful deal in Boston when the world suddenly descends into chaos. A mysterious signal transmitted through cell phones, soon known as the Pulse, transforms anyone using a phone at that moment into a violent attacker.
Clay escapes the initial devastation because he does not own a cell phone. Desperate to reach his wife and son in Maine, he joins forces with fellow survivor Tom McCourt and a young girl named Alice as they make their way out of Boston.
But the people affected by the Pulse are beginning to change. Their violent behaviour becomes increasingly organized as they gather into groups, or flocks, under the influence of a figure the survivors call the Raggedy Man.
As Clay and his companions travel north, they hear about a place called Kashwak that may offer safety from the growing threat. With civilization collapsing around them, Clay must survive long enough to reach Maine and learn what happened to his family.