Nope it has a story and it's actually quite good. There are 4 level areas, at each you build a persistent base which you have to defend from ever increasing more difficult enemy types and waves. The missions on the maps are non persistent, but you still build defences.
I loved it a lot! Spend many hours on it. I liked using traps more than my weapons. Made extensive tunnels where the zombie like creatures needed to get thru, you had punching traps, freezing traps and so many others. The trick was to punch them back towards the reaps that were reloading. You could see their health bars, looked like a pinball game, saw that health bar fling back and forth with less health over time. If some managed to survive my trap tunnels they were shot still. Haha, lovely stuff.
But once the other mode was added and became so popular they spend more and more time on that so development was slow in the end which is why I stopped playing..