The longer missions offer linear but multi-path levels, where you're going to traverse a few different streets on the way to the goal.
They often appear from a multitude of black hole doors, so you can get ambushed from behind at any time. Enemies can spawn anywhere – at the gates, or inside the compound – making for a hectic brawl rather than something you can plan for. The annoyance here is that enemies spawn randomly, and whatever few defensive items you deploy (barbed wire or barricades) make little difference. In shorter missions, your task is to protect the base from an assault. Each mission is a standalone experience, and can last anywhere between 10 and 30+ minutes, depending on the type. The campaign is progressed by undertaking individual missions that appear on the city map. The narrative is lackluster and the dramatic events, that you hear about during cutscenes, offer no emotional weight. This is a multiplayer focused title, so expecting an engaging story is probably a mistake, but the game does so little with the narrative that it could have easily been a generic zombie title. Before each story mission, you get a brief stylized cutscene that describes what you're about to do and why – rescue an important survivor, steal supplies, push back against an assault, and so on. Players follow a group of survivors who are trying to make it in a zombie-infested Washington D.C., while also fighting back hostile human groups.
Despite having the name on the box, Overkill's title doesn't utilize any characters or events from the universe.