A team of electricians from the Chicago power company -- the Dumb One, the Nerdy One, the Black One, and Richard Grieco -- investigates an abandoned building that's causing mysterious spikes in the grid. They assume homeless squatters have tapped into the building's electrical system and damaged something. Instead, they find a hidden nuclear reactor in the basement. Realizing they're out of their depth, they call in a NEST team and go for coffee mess with it. This opens a mysterious portal, which Dumb Guy and Black Guy fall through, forcing Richard Grieco and Nerdy Guy to go through and try to bring them back.
They find themselves in a mysteriously deserted alternate Chicago where it's night and everything is covered in cobwebs. On the positive side, the abandoned things include an armored car full of bags of money. On the negative side, as Dumb Guy capers with joy at their newfound wealth, he is eviscerated by -- as you've probably already guessed from the title and the cobwebs -- zombies. The survivors scream like little electricians and run away. Nerdy Guy and Black Guy jump through the portal, which collapses behind them, stranding Richard Grieco in spider- zombie-infested alternate Chicago.
Realizing they have to rescue Richard, Nerdy Guy goes to work restoring the portal, while Black Guy calls in a NEST team. Unfortunately, instead of leading with "unstable nuclear reactor in downtown Chicago," he leads with "zombie-infested alternate universe," causing NEST to dismiss him as a nutcase. Undaunted, he goes on to give the full story to the military, police, FBI, CIA, Homeland Security, and the fire department, with similar results. Points for honesty, not so much for smarts. Disgusted, he goes to Wal-Mart and buys a bunch of guns.
Meanwhile, in alternate Chicago, Richard Grieco runs into a bunch of survivors. First they want to kill him, but then they see his shoulder tattoo, which marks him as The One Who Was Foretold, so they decide to take him to their leader. On the way, they are ambushed by more zombies and--FINALLY!--a giant CGI spider, which they fight off. Back at resistance HQ, they meet the grizzled resistance leader (some guy you swear you've seen in something else but, according to IMDB, probably haven't) who turns out to be, big surprise, the scientist from our world who created the portal. The portal allows travel between worlds but, unfortunately, is rather unstable and the scientist didn't have the foresight to leave someone behind to reopen it when it collapsed while he was in the alternate universe. Even more unfortunately, it also weakens the walls of the destination world, which allowed other portals to open to the spiderverse, resulting in a spidervasion and, eventually, the spiderpocalypse. The spiders capture humans, either to feed on or to turn into spider zombies. Unfortunately, the spider zombies don't last long, and the spiders are running out of humans. Looks like they need a new universe to invade. The scientist always knew help would come, though. And he'd recognize it by its tattoos; the one difference between the two universes being that tattoos were never invented here.
Nerdy Guy and Black Guy re-open the portal and go through. After some searching, they make contact with Richard Grieco and the resistance and lead them back to the portal which has, of course, collapsed in their absence (guess who ELSE didn't leave someone behind to re-open it). Never fear, though, these guys are electricians--AND they brought the scientist's notes through the portal. They can build a new portal, if they can just keep the spiders from attacking long enough. Even the Wal-Mart gun buying binge turns out to have been a good idea since, unlike most apocalypses, the collapse of civilization and manufacturing in the spiderpocalypse eventually resulted in the survivors running out of ammunition and reverting to bows and arrows. A new portal is constructed, spider zombies are fought, the scientist sacrifices himself heroically, the Black Guy dies less heroically, and Richard and the resistance girl kill the spider queen and flee through the newly-constructed portal. Do you see the flaw in this plan? They don't.
