It seems that there's a growing trend for studios trying to break down the wall between single-player and multiplayer gaming. 2K's Borderlands was the first title in recent memory to have a pop, allowing players to drop-in and out of each other's loot-obsessed romps. The thing is, Borderlands only supported a maximum of four players; it didn't really break down the wall, it only dented it with a small hammer. Then there's Brink, the as-yet released FPS title from Splash Damage, where the single-player campaign and multiplayer deathmatch are one and the same. We don't know how much damage that'll do yet, but it certainly talks a good game. It's like your lazy brother standing in the yard with a sledgehammer. "Oh yeah, I can knock that wall down!" he says. "But not until some point next year."

And now there's MindJack, the new third-person shooter from Square Enix and feelplus Inc. I'm not quite sure what MindJack would do to our metaphorical wall, but I bet it would do something pretty weird - like turning all the bricks into warm, sweaty cheese. At any rate, I certainly know what it's done to me: it's made me very confused. Strangely, I'm quite happy about this. In these innovation-wary times, mainstream oddities like this are comparatively rare.

Now, let me see if I can get this straight. As the game's gloriously cheesy E3 video explains, MindJack is set in the year 2031. The world's governments have apparently "lost their sovereignty" (i.e. they've gone down the toilet) due to the appearance of The Mindhackers - people who have the ability to possess pretty much any person or machine they desire. How they precisely do this isn't clear, but I think it involves the manipulation of some kind of Matrix-like super Internet and/or electronic ghosts (which, as we all know, are the best kind). The bottom line is that The Mindhackers can play with what they like. If they want to take your gran for a test drive, they will; if they decide to possess your blender at the exact moment you're loading it with papaya... well, let's just say that gloves won't feature highly in your winter wardrobe.

The future, as it turns out, is blue and shiny. Some kind of conspiracy is erupting, big evil corporations are to blame, and the only man who can save us is Jim Corbin - a clean-cut hero in the style typical of Japanese action titles (Western heroes, lest we forget, are ugly and balding). In feelplus' E3 demo, Jim and his nubile partner Rebecca are found at an airport, trading bullets with heavily-armoured soldiers. There's a cover-to-cover system, a recharging health mechanic, and a satisfying set of melee attacks when someone gets too close. So far, so generic - but here's the thing about MindJack: as you play, you have the ability to possess pretty much any NPC or enemy you can find. Not only that, but other players can "hack" into your game and also join in the corporeal-kidnap tomfoolery - either as a friend, or as a foe. (See? I hadn't forgotten about our wall. It just took me a while to explain.)

Here's how it works. At one moment you're Jim Corbin, a heroic Chris Redfield clone with a pistol in your hand. Someone starts shooting at you, but you don't fancy poking your head out of cover - so you jump out of your skin and start roaming the level as a sort of sci-fi spectre. At this point the action switches to a first-person perspective, distorted with a pronounced fish-eye lens effect. You float around the map, invisible and free from danger, until you find a suitable host - perhaps an enemy soldier. You assume control of your meat puppet, and suddenly you can make him run about and blast his buddies. When he gets killed, or when you get bored, you can possess someone else: a floating robot drone maybe, or a balding tourist in a Hawaiian shirt.

"Hack into the minds of citizens and make them allies!" proclaims the trailer's narrator, cheerfully. That's one way of putting it - but they're not really "allies", are they? "Helpless victims used as non-consensual meat-shields" would be a more accurate description. Never mind, it's no skin off Jim's back - not unless you fail to protect his helpless, paralysed shell. Wonder too far, and a foe may sneak up on your original body. While it's tempting to view the Jim-torso as just another set of fleshy clothes - perhaps your Sunday best - the reality is that the game ends if his body dies. A blasé attitude to self-preservation simply will not do.

In addition to the AI characters, your major worry will be the presence of other human players. Up to five other people can join your game, opting to either play as a blue hacker (which is good news for you) or as a red one (which may mean tears before bedtime). Whether they choose to help or hinder you, each participant has the ability to roam about possessing people. Furthermore, when they grab a host body its statistics will vary in accordance with their current rank. If they're new to the game they'll be limp-wristed and weak; if they're a veteran, they'll be a formidable force - and in the case of the latter, you better hope they're fighting on your side. Any XP earned while visiting someone else's game will be transferred back to your own campaign, levelling your character and earning you new abilities, so there's a motive for taking part beyond the basic desire to help or grief a stranger. The final details haven't been solidified, but the developers are currently exploring the idea of letting all five hackers oppose you at once - a situation that would surely make life miserable for the lonesome hero.

There are a great many questions hanging over MindJack. Who exactly are Jim and Rebecca? Why were they at the airport? Who were they fighting? And if you're free to roam the level as a ghost, can you not just possess a guard standing near the exit and then do a runner? In the last case, at least, we have an answer: you have to kill all the enemies nearby to progress. Besides, following this plan would mean abandoning Jim's carcass, and as we've already covered, that's a big no-no. With regards to the other questions, I haven't a clue. This is the kind of game that likes to keep you on your toes. You just about get to the point where you roughly understand what's going on, then a giant robot gorilla turns up and starts screaming (and yes, this does actually happen).

In concept at least, MindJack certainly seems to offer a refreshingly unconventional tang. The possession thing has been done before (Messiah and Geist, to name two efforts) but no-one's had a crack recently, and the online co-op/competitive aspect seems like neat idea. Lord knows what the plot will turn out to be like; at the moment the story seems as muddled and mysterious as the end of Akira, and I'm slightly worried that the game will lose something when everything is fully explained. We'll find out either way later in the year.

MindJack will be released on PS3 and Xbox 360 in October.