One year after the start of a deadly crisis that engulfed the planet, Elizabeth Marter and Thomas Edgewater find themselves forced to fight for the future of the human race. In a high-tech facility on the outskirts of Philadelphia, Project Atherius is ready to be unleashed. But at what cost?

As she tries to persuade her father to help her, Elizabeth discovers that he has quite different plans. Maxwell Carver is on the brink of achieving all his goals, but he has one final offer to make. As his plans begin to unfold, Carver reveals the truth about Project Atherius, and about the mysterious experiments that about to be launched into space.

Meanwhile, Thomas and Martha are desperately trying to find a way out of the facility. All they want is to get home, but as the countdown begins they're plunged into a desperate race for survival. A zombie is loose, and so is a madman, and Thomas is convinced that Elizabeth has the answer. Will he find her before it's too late, or will the world end before he has a chance?

Day 365: The Final Day is the thirteenth (and final) book in the Mass Extinction Event series. Readers are advised to start with the first part of the series.