Resident Evil has been scaring the crap out of gamers since 1996. At first, it was the horrific controls that scared the savvy gamer, but eventually (with Resident Evil 4) the series figured out how to make things challenging and terrifying without resorting to cumbersome control schemes. RE4, by coincidence, introduced a minigame called Mercenaries in which the player's usual constraints around ammunition, and the game's focus on narrative were both lifted. What's left is an action game with the infected (we'll just called them zombies from now on), lots of ammo, chaotic gunplay, and all set in the familiar environments used in the main game.
That, pretty much, is what The Mercenaries 3D is; the minigame as a stand-alone title, with a stack of missions and online multiplayer.
It's also, as you may or not be aware, pretty controversial. As we revealed back in June, the game features a rather surprising (and deliberate) feature: there's only one save-game and you cannot delete it. Essentially, this makes the game significantly less valuable as a trade-in (which is why it's rumoured EB Games aren't going to stock it on either side of the Tasman) and, if you do pass it on, whoever plays it next will have to pick up where you left off.
That's not really that big of a deal. There's no story and, once you've unlocked the missions, the idea is that you keep replaying them anyway (there's a heap of skills to find and level up). But it's still a bit stink, which has been the overwhelming reaction worldwide - which in turn is why Capcom have already admitted it was a mistake, and confirmed they won't be doing it again.
The gameplay is, as discussed, a fairly simple affair in which you wander around with your chosen character (assembled from various RE titles) and blow away the (rather stupid) zombies. Ammo is less of a concern than in the main titles, with plenty of the stuff available for all but the truly dire shots. Completing missions opens access to more and you can also unlock characters, costumes, and equippable skills which you can then level up by continually playing the game.