The first season started amidst the zombie apocalypse, where prisoners were being experimented on in hopes of finding a zombie virus cure. One of them, Murphy, is bitten by zombies that raided the facility.
One year later and Murphy, guided by Lt. Mark Hammond (played by Harold Perrineau from Lost), come upon a survivor's camp in upstate NY. Hammond reveals to the small group there that Murphy survived being bit by zombies - thus he is the key to a cure.
Through all this, the group was aided by a military official calling himself "Citizen Z." The sole survivor of a Northern Canadian outpost, Citizen Z still had access to computer networks and communication devices that were still working. He'd intermittently connect with the survivors to help guide them to California.