I fell in love with Metal Gear Solid V: The Phantom Pain the moment I tranquillised a goat and then kidnapped it. The latest title in Konami’s 40m-selling stealth action series is set in a remarkable open world loosely based on 1980s Afghanistan, and wildlife dots the landscape. The animals are beautifully realised and, because this is a video game, they can also be collected.

To capture my caprine victim, I crept forwards on my belly, lined up the perfect headshot, and poomf! down it went. I attached a cord to the prone beast, from which a giant balloon inflated, and lifted it a few feet in the air. At this point the goat woke up, looking pretty startled, and had a few seconds to hang there eyeballing me hatefully, before the balloon shot off into the sky with its cargo. The sound effect, a strangled cross between a bleat and a scream, trailed off as it disappeared into the heavens (where a plane would pick it up).

War has changed.

Say what you will about Kojima Productions, the studio behind the Metal Gear Solid series, but no one could ever accuse them of lacking humour. Over two days with a preview build of MGSV, I not only kidnapped goats, but managed to bag sheep, a bird (which lead character Snake unfussily pocketed rather than wasting a balloon on), and eventually found myself isolated in a canyon facing a bear. It took 12 tranq rounds to the face, and a lot of Benny Hill-ing around the uneven rocks, before finally settling it down. This time the balloon lifted it up by one ankle and, as the bear’s sad face turned towards me, I almost felt guilty. Then it shot upwards with a surprised yowl, and all I could do was smile.

Such details may seem like trivialities when discussing MGSV, one of the biggest sequels of the decade and now one that bears an extra weight of expectation: though no one is quite sure of the details, series creator and presiding spirit Hideo Kojima has become estranged from publisher Konami and finished the game on contract. This will be his last Metal Gear entry in charge.

MGSV has an importance even beyond this and its (very sizeable) niche: it promises a new take on the open-world genre. We’re familiar with the concept from games like Grand Theft Auto, but the term is a general one – referring not to scale so much as potential. The promise of an open-world game, unachievable though this may be, is that players can go anywhere and do anything. In essence, the player chooses how to play. This is why, though GTA has a campaign, when you talk to people about GTA it’s never mentioned. What is brilliant about that game is how it allows players to create their own stories. And so with MGSV, except with one difference that changes everything.

MGSV’s open world is not enormous. It’s not a sprawling Just Cause-type map that requires jet planes to transverse. But this alternate-universe Afghanistan is dense in a previously unseen way, not only packed with stuff but underpinned by an integrity of structure that means everything is a prop for interaction. Open-world games always have to compromise between scale and local detail – which is why GTA’s core gameplay is such a masterstroke, because it encourages driving. MGSV is the first open-world game that feels like every rocky outcrop, every bush and every patrol path has been hand-placed and then tested and then tested again for the possibilities it offers. It is a curated world of both wide open spaces and quite extraordinary density, where the player’s opportunities are greater than ever before.

Take a simple example – buildings. In most open-world games there are buildings you can enter, and buildings you can’t. In the majority of cases, entering any interior requires an immersion-breaking loading screen. Not in MGSV. It is able to do this because, of course, there are fewer buildings in an Afghan settlement than a given patch of LA, but even so, the effect on the player is tremendous. This is integrity of design: the idea that everything in a world is a physical presence that reacts as it should, rather than a solid 3D shape that’s basically wallpaper. So used are we to the disappointment of locked doors, so inured to the possibility that things could be different, that the simple fact you can seamlessly enter any building you see in MGSV strikes with the force of a revolution.

It is not just about doors. MGSV’s Afghanistan is populated with small settlements of Russian troops and larger bases, while the paths between them are patrolled and often broken up with checkpoints. The intelligence of computer-controlled soldiers in this series has always been a brilliant balance between predictable movements, patrol paths and so on, and the unpredictability of how they react upon Snake being spotted. If they call it in, they’ll trigger an “Alert” phase, before beginning to hunt the player down en masse. This AI flip between a sedate, controllable state and an aggressive pack mentality is not just a classic mechanic, but the underpinning to the whole game’s concept: if you didn’t care about being seen, then everything’s a bust.

Metal Gear remains a stealth experience – it’s always better to sneak into enemy territory rather than stroll up with a machine gun. Photograph: Konami

In MGSV, the AI has been improved enormously, and the greatest change is the addition of unpredictable movements when soldiers are not aware of Snake. In previous games you could watch them perform the same patrols again and again. In MGSV they have patrol paths, but within these they’ll pause at random points, occasionally chat to each other, and have a terrifying propensity to check over their shoulders just when you’re creeping up from behind.

Get spotted and the game will move into “reflex mode” for a few seconds: a slow-motion sequence where you have the chance to silence them with your fists or a gun before they raise the alarm. Though a great mechanic in isolation, and a safety net for careless play, the brilliance of reflex mode is that you’re often unprepared for it – and a missed shot or a reload is disastrous. If an alert goes up, the soldiers swarm intelligently around the player’s position, never moving in straight lines but taking cover, attempting to flank, and doing everything they can to maximise the advantage of numbers.

In these scenarios the enormous improvement in MGSV’s control scheme comes to the fore. In the previous games, controls were not optimised for shootouts, making any large-scale engagement a scary prospect. The theme of MGSV, the gradual disintegration of Snake’s moral core and mental state, means that killing no longer feels like a last resort so much as giving these scumbags what they deserve (though the game still emphasises stealth as the “best” way to play). To this end, Snake’s potential armoury is enormous, and combat features gorgeously smooth switches between the “standard” third-person camera and an over-the-shoulder aiming mode (also present in MGS4 but not nearly as slick), while movement in general has been made as unfussy and responsive as possible.

Kojima Productions has the ability to reimagine mechanics without losing their core purpose. In MGS3, for example, Snake could be seriously wounded, and the player had to enter a menu screen to perform basic first-aid tasks like removing a bullet or disinfecting a wound. In MGSV, the idea of serious injury remains, but now there’s no need to jump out into a menu – the player simply holds a button while Snake takes care of it. This may sound like cheapening the mechanic but it’s the opposite: the point is not one button press versus several button presses and a menu, but the necessity to get out of combat and find 10 seconds to take care of the wound. The mechanic keeps its place, but is woven more naturally into the world, and contributes in the same way the doors do to an experience of seamless integrity.

Perhaps the most brilliant manifestation of this is Mother Base. This is the offshore private militia that Snake is rebuilding, and everything about it ties into the world of Afghanistan. That bear I kidnapped? It went back to Mother Base to join my menagerie. Kidnapping guards eventually results in them being “turned” to your side and staffing Mother Base, after which they can be assigned to different departments. These units can then research new items or bonuses to help you in the field, and all of this is happening in real time – so it’s possible to kidnap a soldier, then a few minutes later assign them to R&D, and use the increase in stats to research a new gun. Then you can call in your helicopter to drop off the new toy. Hell, you can call in the helicopter and make it drop the package on top of a guard’s head just for a laugh.

And if you’ve found the right tapes, dotted around MGSV’s encampments, and researched helicopter speakers, you could have the chopper blasting out Hall & Oates or A-ha or Kim Wilde for maximum impact – just in case you forgot this was set in the 1980s. Such experimentation has always been the lifeblood of MGS and here the possibilities are more numerous than ever before – Kojima Productions specialises in thinking through every possible interaction a given item or situation could lead to, and making sure it’s catered for in the game. Guards can be pushed and pulled around by any number of distractions, rendered paranoid by continual mysterious noises, and even freaked out by a dog’s barking.

Snake can call on the help of various ‘buddies’ in the game. One is, of course, a dog. Photograph: Konami

The dog is one of several “buddies” Snake can acquire through MGSV, one of which can accompany him on each deployment. You begin with a horse, soon enough find a puppy (that quickly grows up into a soldier’s best friend), and can soon go in with a human companion who will snipe enemies and infiltrate encampments to provide Snake with information. A particularly neat touch is the ability to “hang” on one side of the horse, meaning you can slowly trot it past guards who only see a lost animal wandering around. Such depth means that, while the open world offers a challenge unlike any MGS yet, Snake’s range of options is exponentially greater.

MGSV feels like a game of endless possibilities. MGS as a series has managed one of the industry’s greatest feats, which is to have remained at the cutting edge over four console generations. No other AAA title has managed this. What is even more remarkable is that Kojima and his team have managed to maintain the core elements of the series over this time, and the way they work in MGSV is a revelation.

The heart of MGS is remaining unseen. Whereas other games cast you as the vengeful hero or the central character all others respond to, the dream playthrough of any MGS game is one where you’re not spotted once (and each contains special rewards for this). The fact that MGSV manages to maintain this spine of play while adding so much depth of interaction and scale is nothing short of astonishing. I’ve never played in a sandbox like it, and Afghanistan is only one of several locations (the others are as yet unannounced).

There is so much more. The day/night cycle alters guard patrols and reduces their vision, changing the world in a real way and encouraging different approaches. One mission ends in a boss fight that I ran away from, while a colleague blew them away – and MGSV was fine with both solutions. You can steal enemy gun emplacements and stick them on Mother Base. You can explore Mother Base, improve your troops’ morale by chatting to them, and on your real-life birthday get a nice surprise.

You can train recon teams to feed real-time information about enemy patrol paths, turn your extraction chopper into deadly air support that strafes enemy bases while playing Ride of the Valkyries, or train up the horse to defecate on command – which can cause enemy jeeps to lose control. Then you can nick that jeep and drive slowly through an enemy base, and no soldier will think to look twice at who’s behind the wheel.

MGSV promises so much, and after two days in its world it feels like, if anything, it’s going to over-deliver. If this is to be Hideo Kojima’s last Metal Gear Solid then all you can do is salute, and acknowledge that despite the sadness – what a way to go.