There's a new threat to civilization, and this time it's something we can't see: the Nanostray virus, which is deadly enough to serve as an immediate threat to humanity as we know it. The solution? Well, as anyone who's seen Fantastic Voyage (or, erm...Innerspace) knows, it's time to shrink a spacecraft down to microscopic size and take out that virus ourselves!

That's the plot to Nano Assault, and it fortunately doesn't get in the way of the game's action. In fact, apart from a respectably brief opening sequence, there's little said about it at all. The game wants to get us shrunk down and blasting baddies as quickly as possible, and we certainly won't complain about that. Of course, this isn't the first time that developer Shin'en have brought the microscopic virus to our attention: it originated in its Nintendo DS shoot 'em ups Nanostray and Nanostray 2.

Each level takes place upon a single cell. The particular missions vary, but they're always some combination of collecting healthy DNA strands, clearing out small manifestations of virus, recapturing contaminated areas, exploring tunnels and fighting massive — well, relatively massive — bosses. You maneuver your ship with the Circle Pad and fire in any of four directions with the face buttons. L changes the spread of your shots and R fires your secondary weapons, once you've unlocked them.

It won't take you long to be utterly dazzled by the sheer graphical scope of Nano Assault. Each individual cell is clearly distinct from any other, and they are each impressively rendered and beautifully detailed. In fact, everything about the game looks great. The screenshots don't do it justice; this is a game that has to be experienced to be appreciated, and it's clear that a huge deal of effort was put into making Nano Assault as graphically impressive as possible.

In the same regard, the 3D effects are absolutely stunning. The depth of field is remarkable, and everything from level elements to boss attacks make great use of the console's 3D capabilities. In fact, if you were looking for just one game to show off 3DS's graphical abilities, you wouldn't go wrong in choosing Nano Assault.

The aforementioned tunnel sequences in particular rely on the 3D effect to help you dodge and weave your way through waves of enemy fire, and they're handled gorgeously. There are a few unfortunate instances of slowdown, but if anything they just serve to remind you that with the sheer volume of motion on the screen, it's a miracle there isn't much more of it.

It's primarily played with an overhead perspective, and as you circle the small cells you'll find the entire structure rotating beneath you, Super Mario Galaxy style. As you clear cells of infections you'll need to keep on your toes — this is one vicious virus, and you'll be bombarded with waves of enemies and projectiles from every angle.

For this reason, Nano Assault is not for the faint of heart. You need to keep moving constantly, spraying the air with bullets in all directions. Apart from the often clever boss fights, there's no time to think, as the smallest hesitation will get you killed. This treacherousness is further compounded by the annoying tendency of enemies to spawn directly beneath your craft, resulting in an instant kill. That's a bit less fair a move than we'd hoped to expect, but, again, if you keep moving it shouldn't result in more than the periodic annoyance. It's still a questionable programming choice, though.

The tunnel sequences are where the game really shines. As your ship is sent off to explore unknown areas, the perspective changes to a behind-the-ship view, with only straight ahead firing possible. As your ship drifts and drops through networks of cellular corridors, it's up to you to guide it past obstacles and gun down the enemies that appear along the way. These sequences are always innovatively crafted, with smooth difficult curves and inventively cruel enemy placements. It's likely that these levels will take several passes to master, but once you do make it through alive, the feeling you'll get is one of pure satisfaction.

Nano Assault is, make no mistake, an extremely difficult game. One hit kills and relentless waves of enemy projectiles all but ensure that you won't conquer this viral invasion easily. That's part of the appeal, though; while Nano Assault is always a challenge, it almost never feels unfair. It's an addictive experience with some of the most satisfying battles we've had on the console yet, and for fans of the genre, it's far from a misfire.

Completed levels can also be replayed with additional challenges to complete (such as succeeding without dying — which is never easy!), with high scores uploadable to online leaderboards.