The story begins in West Coast Avengers 3.
Hank Pym has given up on being a Superhero himself but still wants to lend his support to the Avengers so we get this:
Then it all starts with a mysterious phone call:
This incident is repeated in Issue 4:
West Coast Avengers 5:
While Wonder Man is busy filming a movie and the rest of the Avengers are off to the Land Within (the Cat People's realm) Hank decides to meet up with Ultron in WCA 6:
WCA 7 opens with a female Red Cross worker finding the head of Ultron-11 in the ruins of Alicia Masters' old apartment.
She takes it home just to plop it on her TV set. Of course the head sparks to life, takes over the TV and hypnotizes her.
Meanwhile the Ultron 11 head has been delivered to a computer expert in
Hank and Mark end up in the very same lab...
The robots continue to battle and Ultron manages to rip Marks head off and then erases all records of him from the computers.
He turns to Pym and rather than killing him on the spot he wants to teach him "the definition of cruelty."
Mark it turns out isn’t completely gone yet and he manages to will his body to come back to him.
He shows up on set of Wonder Man’s movie with his head attached on backwards. He can talk Simon into coming with him to the lab to save Hank, who Ultron hooked up on some sort of draw-and-quarter machine, just about to tear him apart.
Wonder Man smashes through the wall and starts battling with Ultron, overcoming his fears and finally destroying the robot.
But what about Mark-12?
We get a bit of an aftermath in Issue 8: