*This contains major spoilers for all of Part 1 and Part 2. If you haven't read them in their entirety, I highly recommend you go back and finish it. If you have finished, this should help refresh your memory before we move on to Part 3*
Several years ago in a shady lab somewhere in Central England, Troy Schaefer, former member of the British Army and amateur scientist, teamed up with his colleagues to create the Ambrosia Project, an experiment involving nanotechnology that permits its host superhuman abilities such as super-strength and an enhanced healing factor. Unfortunately, small flaws within the Ambrosia nanites along with the brutal mistreatment of the experiment's test subjects concluded with the Ambrosia Project's failure. Planning on hiding the truth, the test subjects were simultaneously executed via gas chamber, only for the corrupt Ambrosia to prevent them from dying. As a result, they began reanimating as zombies with inhuman strength and a healing factor so advanced, major blood loss would do nothing to stop them. But the problem truly came to light when the reanimated corpse of a test subject known only as Ares developed the mysterious ability to manipulate and weaponize his own blood, earning the insidious alias Bloodletter. His rampage on the facility that created Ambrosia spread to the outside world, unleashing the Deadmen on the rest of unsuspecting England.
Meanwhile, four orphans named Aaron O'Connor, Jonah Franklin, and twins Nika and Natalya Koslov grow up together at the Hecox Home for Children, their lives altered forever once the Deadmen arrived. After saving a young street urchin named Katrina "Kat" Mathis from the oncoming wave of Deadmen, they witness the death of their caretaker Roderick Hecox, who sacrificed himself to blow up some of the invaders. The kids are later evacuated to safety by Schaefer, who now leads a paramilitary group known as the Anti-Infection Squad in order to hide the truth.
While the rest of the country (and possibly the world) began to fall apart at the rise of the Deadmen, the children took refuge at Matheson Babel Tower, a tall structure hidden in the Forest of Bowland far away from the larger concentrations of Deadmen. Following the death of their remaining caretaker Agatha Hecox, three of the children (Aaron, Jonah, and Kat) dedicate themselves to rising up the ranks of Schaefer's reestablished paramilitary group, the Dead Zone Infantry. Gaining knowledge in both weaponry and the very threat they seek to destroy, they are able to hold their ground among the DZI until one fateful day. On Aaron's fifty-third mission, poor planning during the reclamation of Longridge leads to the deaths of many DZI officers at the hands of the infamous Abnormal known as the White Scythe. Held responsible for their deaths, Aaron is discharged from the DZI for good.
Depressed and feeling purposeless, Aaron O'Connor began noticing possible signs of infection after the White Scythe's blood had entered a wound in his hand during the Longridge mission. However, he couldn't confirm his infection until he uncontrollably mutilates the hand of fellow DZI officer Rupert Cromwell and after realizing he could communicate with the Deadmen themselves, even forming an acquaintanceship with a surprisingly passive Deadman apparently named Harry.
Through his newfound alliance with Harry, he meets other Deadmen who mean him no harm and discovers the hiding place of Bloodletter, an abandoned village deep within the forest referred to as the Dead Center by the local undead residents. However, Bloodletter proves himself to be more hostile than the Commoner Deadmen, even attempting to kill Aaron multiple times. His attempts only speed up the process of Aaron's turning due to rapid blood loss, and he only cared about learning the location of Troy Schaefer, who had killed him and his daughter during the Ambrosia Project.
Seemingly only a day or two away from succumbing to the Deadmen infection, Aaron does what he can to hide his secret but fails, having been caught and arrested by the DZI once he returns to Matheson after spending another day in the Dead Zone. Now spending his final hours in a prison cell next to his old friend Natalya and a psychotic woman with a shady alliance with Schaefer named Lavinia Dyer, he is forced to reveal the truth before his scheduled execution.
Taken to the open gates of Matheson, Schaefer prepares to kill Aaron for good, both for his crimes and to prevent him from turning. However, Aaron's deep desire to leave his mark on what remained of the world motivate him to fight back, quickly disorienting Schaefer before fleeing into the Forest of Bowland with Harry and his other undead allies.
After momentarily breaking down from exhaustion, Aaron wakes up in front of an abandoned laboratory next to Harry and their two Deadman friends, Jade and Kyle. According to Harry, this facility is the place where it all began.
The epicenter of the world's end. And quite possibly, Aaron's final location...