When you first jump into Zombie Apocalypse, it's going to seem awfully shallow. And it is. At first. But as you play day by day through the game, something changes.

After the jump, find out what happens 28 days later.

You're going to have soldier your way through the default mode, sliced into days which progress through the same seven levels over and over, gradually layering in each new zombie, weapon, and game mechanic. It builds slowly and it takes a long time. 28 days later, you'll finally get the last weapon. A terrible boss fight comes a few days after that. And then you've still got 25 levels to go.

Fortunately, you'll have unlocked a few options for how to play. You can start at later levels. When you die, you can continue as often as you want, although it resets your score. But once you get to "7 Days of Hell" mode, you're going to be all set. This is where Zombie Apocalypse unfolds all its frantic rotting charms, pitting you against far too many zombies at once, dropping all manner of guns in your lap, and mixing various zombie types into insidious combos. "7 Days of Hell" plays through a black-and-white filter that's unsettling in a completely different way from the full-color gore-drenched default mode. This is the Zombie Apocalypse as it should be. Nonstop carnage. A bit overwhelming. A bit retro. You're never sure if you're getting the grenade launcher or the sniper rifle. You're knee deep in the dead.

The blueprint for this game is clearly Robotron, Smash TV, and Geometry Wars. One control stick moves your character, the other fires your weapon. The gameplay is a matter of different kinds of zombies pitted against different kinds of weapons. Sometimes you whip out the chainsaw to get yourself out of a fix. But mostly you just shoot. Your default weapon is an assault rifle with limitless ammo.

You can walk over power-ups that give you a better weapon for a certain number of shots. The shotgun is easy-peasy, of course. Who wouldn't want a shotgun when dealing with zombies? Molotovs require a bit of finesse. The flamethrower is a thing of beauty. The rocket launcher is fine for random mayhem, but it fares best if you use it as surgical tool. The chaingun? Well, the chaingun is just a great big middle finger to a couple hundred zombies.

An important nod to Robotron and zombie lore are the occasional survivors. They'll emerge from the zombie spawn points, calling for your help. When you touch a panicking survivor, she holds her position while a helicopter searchlight tries to pinpoint her. If you defend her until the chopper drops its ladder, you get a mess of points and a teddy bear. Yes, a teddy bear. Zombies flock to teddy bears, so they're wired to explode, much like the pipe bombs in Left 4 Dead. Consider them a "Get out of Hell Free" card.

Oh, and if you don't defend the survivor, she turns into a super fast, super aggressive zombie who's particularly angry at you. You can't blame her really.

You don't have health in this game, but neither do you die instantly. When zombies get their Grabber McNasties on you, you have to jiggle the left analog stick to shake them off. One zombie is no big deal. Neither are two. Three zombies are even shakeable. But the problem with zombies is that they travel in packs. Where there is one, a second usually isn't far behind. So while you're busy shaking off that first zombie, the second zombie joins the grapple. And when you're shaking off three zombies, there are likely a fourth, fifth, and sixth shambling up. Time and math are always against you. It's the way of zombies.

As with any arcade shooter, the core of the gameplay is your score. Zombie Apocalypse is a typical multiplier-based shooter. For every five zombies you kill, your multiplier goes up by one. When you die, in a fate worse than death, your multiplier drops back down to zero. You can do a risky chainsaw move on lone zombies to immediately boost your multiplier by three. It's a great risk/reward calculation where the gratuitous bloody chainsawing animation is part of the reward. There's support for up to four player simultaneously (online or in the same room), but your scores are shared. Which is as it should be. There's no room for personal glory in a zombie apocalypse. That way lies disaster. If you don't stick together, those things will get you. It's the way of zombie survivors.

Zombie Apocalypse uses leaderboards to track scores, but it doesn't do it very well. I'm not sure why the closest score from my friends list only occasionally appears in the upper right corner, because it should be there the entire time, Geometry Wars style. Furthermore, the leaderboards seem like an afterthought. Why does the friends list leaderboard show zeroes for all of my friends who don't even have the game? I'm supposed to scroll through all those guys to find my friends who have actually played? And why doesn't my own score appear on the friends list as a point of reference? Am I not my own friend? The developers should know that scores screens are an important part of an arcade game. You can't just treat them like a data dump.

Once you've unlocked the more gratifying ways to play, Zombie Apocalypse is a delightfully action-oriented and frenetic alternative to the more cerebral puzzle-ish Burn, Zombie, Burn (reviewed here). Also, unlike Burn, Zombie, Burn, it's not cartoony. Well, there are knife-throwing grannies and the victory animations are nothing if not childish. There are even pregnant zombie women who hang back and, uh, birth flocks of head-eating bugs. That's probably not a mental image you needed.

But Zombie Apocalypse is mostly earnest, gory, and traditional, full of wonderfully animated maiming and dismembering. It plays out with the same pacing and tone as Left 4 Dead, but seen from overhead. The grappling is particularly well done. I love the death animation when you're overwhelmed, dragged the ground with your legs kicked out and jerking while the hungry zombies pile on. It's downright iconic. If I'm going to lose my multiplier, it's some small consolation to be treated to such a grisly death.

You can download Zombie Apocalypse for ten bucks from the Playstation Store or Xbox Live Marketplace. And if you're a fan of arcade shooters in general or zombie games in particular, I recommend that you do.