One hundred days after humanity was brought to its knees, Elizabeth Marter has joined a small group of survivors in Boston. Under constant attack from scavengers who live beyond the city limits, Elizabeth and the others desperately try to come up with a plan. But when they learn that a more organized enemy is on the way, they realize that they face a battle for what's left of their resources.

Thomas Edgewater, meanwhile, has joined up with a ragged bunch of soldiers who are on their way to Boston. As they wait for their orders, however, Thomas and Toad find themselves ordered to guard a captured zombie. By the time the hundredth day draws to a close, Thomas will have begun to suspect that the mysterious Sarah Carter isn't quite who or what she seems. He'll also have met someone who will change his life forever.

Day 100 is the sixth book in the Mass Extinction Event series, chronicling the collapse of human society through the eyes of two very different people who are gradually being drawn together. Ends on a cliffhanger.