I don't know what an Undying Bear is exactly, but I've vowed to kill it. I hope it's just a name. This is a mission for the island's Rakyat tribe, and Rakyat tradition dictates that I must defeat the creature with the infinite-ammo pump-action shotgun they've given me. A recent tradition, I would guess, but one I'm happy to honour. The truth is, I have an ulterior motive for finding and killing the legend: I'd really like a new rucksack.

A lot of what you do in Far Cry 3 raises perplexing questions: why would a rucksack made from the skin of the Undying Bear hold more than the one I made from four dead dingoes earlier? Can't I just make one out of six dead dingoes? What is it about Undying Bear skin that facilitates a particularly capacious rucksack design? And more to the point: if it's never died, how would anyone know?

But as I scramble away from it, panic-firing my traditional tribal pump action, what I'm actually wondering is this: when did Far Cry 3 become so good?

We'd been told it was an 'open world' game, but everything Ubisoft showed of it made it look like a monologue-heavy, tightly scripted adventure, its freedom limited to small mission areas. That is in there, it turns out: there's an absurdly long series of missions about rescuing your friends from the pirates who've captured them. But it's just one of the many different games you can play on this vast, freely explorable tropical island.

Hunting wild game to make bags out of their skin is another. Guns, money, syringes and all types of ammunition require their own special container, and every size of every container can only be made from the skin of one particular species of animal. And while guns, money, syringes and all types of ammunition are abundantly available on the island, its people have apparently never invented the bag.

So you, American tourist Jason Brody, must bring your container technology to the island by personally inventing and reinventing various types of harnesses, wallets and sacks, culminating in your magnum opus: the Undying Bear Skin Rucksack, a masterpiece of dermatological engineering capable of holding up to 96 leaves.

If you're going to ask players to buy into a system so hilariously removed from its origins in real-world logic, it had better work. It does. Making the island's wildlife the fodder for your personal upgrade system turns you into a hunter, forced to study and understand the jungle as you explore it. The place teems with life, to the point that you'll often just sit in a bush and watch it. Check out the leopard stalking those boar! What are those dogs howling at? Ooh look, a Komodo dragon mauling a villager!

Something's actually happening over there, and you can go and find out what. Maybe steal its skin.

They don't just fight amongst themselves: the island is dotted with pirate outposts, and the roads are travelled by trucks and cars full of pirates, Rakyat rebels, and civilians. Almost any pair of these have some reason to scuffle if they blunder into each other on their randomised routes, and hearing it happen around you makes the place feel alive. Distant gunfire or beast growls are never just ambience: something's actually happening over there, and you can go and find out what. Maybe steal its skin.

Those outposts are what the game is really about, and conquering one demonstrates everything that makes it great. Your first job is to scout: you've got an entire island of free space to circle this small settlement, and the zoom lens of your camera to study it with. The first Far Cry let you tag enemies with your binoculars: once seen, they're marked on your map in real-time. Far Cry 2 ditched that for being unrealistic. Far Cry 3 brings it back with a vengeance: not only does your camera mark enemies on the map, it lets you see them through walls from then on. As with the skin-crafting, the philosophy is clear: screw reality, this ability makes the game more fun. It does.

Once you've scoped and tagged the 5-10 enemies guarding the outpost, you have perfect situational awareness. You could open fire, but at least one of the pirates will make it to an alarm panel. That brings a truckload of goons to reinforce, and things get very messy. So priority number two is to disable the alarms, and the systems for this are deliciously clever.

You can shoot them. OK, that one's not clever, but it has an interesting complication: only the panel you shoot is disabled, and even a silenced shot will make enough of an impact noise to send the guards running to the others. If it's a small camp, and you've scouted it thoroughly, and you're sure you have line of sight to every panel, you can speed-snipe them all before the guards can set them off. This is cool.

Trickier, but cooler still, is to methodically eliminate each pirate without alerting the others. This is tough, but your tools support it: you can lunge for any unwitting enemy nearby and impale them on your machete before they can call for help. A perk system lets you spend experience points to upgrade stuff like this, including a great trick that lets you steal the dying guard's own knife and throw it at someone else for a second silent kill.

My favourite method, though, is often more practical. If you can get to one of the alarm panels in person, you can tamper with it to disable them all . It's silent, instant and comprehensive. But the panels are always in the heart of the outpost, watched by everyone. Getting to one requires perfect scouting, obsessive planning and steady nerves.

That generally means creating a distraction, and that's another thing Far Cry 3 is great at. You have a dedicated button for throwing a rock, and the sound will distract any idle guard in earshot. It's not a new feature for the series, but short-sighted enemies, more predictable AI and the see-through-walls thing make it massively more useful this time. And those same factors apply to other distractions: a car-full of rebels showing up, a stray bear wandering past, or the pirates' pet leopard suddenly finding its rickety bamboo cage shot open.

Last time I did the cage trick, the leopard savaged every pirate in the camp, waited for my Rakyat allies to show up and take over, then savaged all of them too. That camp is under leopard control now. I gave him sovereignty.

Part of what I love about all these systems in Far Cry 3 is the way they chain together. I find myself hedging my bets: I want to take an outpost down undetected, but I'll try to sneak in and disable the alarms first in case I screw it up. And before I do that, I'll drop some C4 under a nearby truck: if I'm close to being discovered, detonating that'll take their attention off me. Often, halfway through carrying out my plan, the guards catch sight of something they want to attack outside the outpost walls, and rush off to shoot at it. So you have to be ready to restrategise on the spot, and sneak through any window of opportunity that opens up.

Once, when I couldn't get to an alarm panel, I was rumbled halfway through eliminating the guards. I finished the rest off before the reinforcements arrived, but that left me trapped in an empty building with eight angry pirates hunting for me. It was heart-poundingly tense. I'd peek out of windows to tag them with my camera, then watch their silhouettes through the walls until one strayed close. I couldn't risk leaving the huts, so I'd just throw a stone near the doorway. The sound would lure him inside, I'd impale him on my knife, drag his body out of view, then wait for my next target.

If you do manage to disable the alarms, your reward is an even more satisfying second phase to the fight. You still have to eliminate all the guards, and it's still good to remain unseen, but now it doesn't matter how panicked they get as their friends drop around them.

Far Cry 2 had outposts too, though they were smaller with fewer ways to approach. They were also the source of my biggest problem with that game: they repopulated. Far Cry 3's solution to this problem is: they don't. You can conquer the whole island, outpost by outpost, turning each into a rebel base with hunting and assassination missions to help secure the area. It'll just take you a while, because it's huge.

Taking over an outpost gets you a new safehouse with a built-in shop, selling a fairly ridiculous array of guns and attachments. These are unexpectedly satisfying to use, and Far Cry 2's slightly tiresome habit of causing them to randomly jam is gone. It's also very generous about which ones you can fit silencers to - I ended up taking a silenced SMG, a silenced sniper rifle, the silent bow, and a grenade launcher for emergencies (leopards, basically).

Yes, it's a game in 2012, so it has a bow. Along with the endlessly distracting rock and the brutally effective machete, the bow makes you feel like a hunter, stalking and butchering teams of heavily armed guards with nothing but blades and guile. You're never forced to get it, and it's not actually as effective as a good silenced sniper rifle, but it gives you a sense of identity the other two games never had. As you walk through a silent town of corpses, pulling your arrows back out of their skulls, you can't help thinking, “Christ, I'm glad I'm on my side.”

You left the mission area! Restart! You lost the target! Restart! You failed the quicktime event! Restart!

Your captured outposts become hubs for two types of missions: assassinations and hunting quests. Both are fun, but assassinations are the highlight: you've got to take out an enemy commander with only your knife.

I've been putting it off, but I should probably talk about the story missions. The pirates have captured - no kidding - you, your brother, your brother's girlfriend, your girlfriend, your friend, your other brother, and your other friend. By the end of it I was surprised we didn't also find my mother, niece and high-school English teacher somewhere in the compound.

It's not all bad. About half of the Jesus Christ thirty-eight missions give you enough freedom to have fun with the predatory combat systems that make the outpost fights so great. The other half... erk. They're like a guided tour of all the clumsiest ways to mash story and videogames together until both of them break.

You left the mission area! Restart! You lost the target! Restart! You failed the quicktime event! Restart! A plot character got themselves killed! Restart! We spawned some enemies in a spot you knew was empty! Restart!

I don't feel like you have to be that smart to predict this stuff won't work. You don't have to play a lot of games to see how it backfires. And you don't have to talk to a lot of gamers to find out how much we hate it when you cheat or punish us to make a scene play out the way the story needs it to. It's so painful to see clumsiness like that in a game that's otherwise so elegantly designed.

The island itself is so rich and interesting to explore that it'd be a fantastic game even without any main story missions. So the question is, does the presence of a half-rubbish campaign hurt it? A bit, thanks to one unwelcome quirk of the level-up system: most of those neat perks, including the knife-throwing one, are locked off until you reach certain points in the plot. That pretty much forces you to play it, though thankfully not for long. Most of the good ones unlock at the same time as knife-throwing, a few hours in. You can safely stop there and get back to the good stuff.

Elsewhere in Far Cry 3's efforts to be all things to all people, it somehow has four competitive multiplayer modes and a separate co-op campaign. Playing this pre-release, it's too soon to review the competitive stuff. The co-op missions are a lot of fun, though: brisk, ridiculous shooting galleries about helping each other plant explosives and repair vehicles. There's no server browser, unfortunately, but they're best played with friends where possible. My favourite moment was taking a stealthy loadout and playing scout for a heavy-gunner friend in a dark cave: I'd 'spot' targets in the dark to highlight them on his HUD, he'd gun them down and draw all their fire.

Another caution about online stuff: Far Cry 3 uses Ubisoft's Steam-like service uPlay, and if you play online, your game can get interrupted temporarily if your connection or their servers go down. It's just a brief pause, though, and you can always start the game in offline mode to avoid it entirely. You miss out on uPlay achievements and a few lame unlockable rewards that way - I didn't particularly care.

Other than that, it's a nice PC version: responsive mouse movement, specific graphics and FoV options, tutorials reflect your custom controls, and it runs decently on Ultra-everything on a modest 3GHz dual core machine with a Radeon HD 5800. The engine doesn't quite suit the jungle as beautifully as it did the African desert in Far Cry 2, but it has some beautiful views.

The original Far Cry's developers Crytek used to describe that game's philosophy as 'veni, vidi, vici': you show up, you scout out the situation, and you decide how to conquer it. Ubisoft kept the Far Cry name, and Crytek tried to stay true to its spirit in the Crysis games. But only Far Cry 3 really feels focused on doing that concept justice. You've got a huge island to explore, ridiculously effective tools for scouting every hostile situation, and so many clever intersecting systems to inspire creative ways to conquer them. It's a better stealth game than Far Cry 1, set in an open world that feels richer than Far Cry 2's. That's an amazing thing to play.