Basically, a company called JC Jenson sent robots called worker drones to a planet to mine resources, however after all the humans were wiped out on that planet (because of a disaster they caused) the workers created their own society
JC Jenson wasn't too happy about that. So they sent killer robots called murder drones to kill all of the worker drones, hence the name, but the majority of them hid in a bunker for months or even years.
(It's not said how long they were in there.)
Uzi (the main character) becomes friends with one of the murder drones after leaving the bunker to find parts for a weapon to fight them.