As bombs go off all over the world, Doc, Roberta and 10K avoid the nuclear fallout by taking cover in a tunnel. For the record: Doc's been shot in the shoulder, nobody knows where Cassandra or Murphy have gone, and Addy and Mack have been out of the picture for a while now.
All of this is happening while Citizen Z prepares to get blown up by the nuke that's about to drop on hm. However, the outpost he's in has its own self defensive system, launching a missile that blows up the nuke in mid-air. Unfortunately, the heat blast thaws out zombies out in the tundra and they start to overrun the base. Citizen Z gets just enough of them subdued before sending an emergency broadcast telling anyone listening to be on the lookout for Murphy ... and, as an extra incentive, announces that the CDC will hand out cures and large bounties for Murphy's safe delivery.
Mack rides up on his ATV to find Addy killing the last of the Sisters of Mercy, who had all gone Z. Apparently that kid banished from the camp on his 13th birthday a while back returned, burned the place to the ground and released the zombie bear. Since neither of them wants to deal with that thing, the two ride off to see what they can find. They decide to get the message out to the others that they’re alive, but their commandeering of a radio transmission reveals that pretty much the entire world is now looking for Murphy ... and that sweet reward money. Also, there’s a strange numerical code that Addy has picked up from her ham radio, repeating over and over ...
Murphy, meanwhile, is doing okay, stripped of his decaying skin and now a rather shiny silver. He's got his posse of zombies and they've all gone shopping at a thrift store when he runs afoul of a guy who wants to take him in for the bounty. That plan goes awry, though, when Cassandra appears out of nowhere and wastes the dude. If you'll recall, Murphy saved Cassandra last season by biting her (or something); she's not a zombie, but she's not quite human, either.
Meanwhile, Roberta is starting to have a bit of a breakdown as she's gone off on her own in search of help for 10K and the wounded Doc. Just before going through with giving herself mercy rather than dying of starvation, thirst or exposure, the sound of a young girl being attacked by a gaggle of Z's gives her reason to live again. She saves the little lady and is taken in by her family, who live a seemingly Little House on the Prairie kind of existence. They give her water and supplies and she meets up with Doc, 10K, Mack and Addy, all of whom have been reunited (and it feels so good).
The gang travels to Cheyenne, Wyoming, where they find Murphy running a zombie strip club. Roberta demands he accompany them to California but all Murphy wants to do is show them the new stripper tricks he's taught the zombie ladies (which are fairly gross). When a bounty hunter named Vasquez arrives to take Murphy, it's sheer Z pandemonium and Murphy and Cassandra escape.
Finally, Roberta and the gang resolve to find Murphy and get him to California, Citizen Z runs off to find help ... and, oh, remember the crazy Sister of Mercy who brought pie and sexual pleasure to Murphy? She's now pregnant with Murphy's baby, roaming the countryside in search of Daddy.