When this new instalment in the Hitman series was first announced back in June 2015, the overwhelming reaction was one of confusion.

Following 2012’s poorly received Hitman: Absolution, the franchise was to re-emerge as something unprecedented in the action adventure genre: a six-part digital episodic release, spread across a whole year, each part constituting one level of the final game. Nobody was quite sure what this experiment would look like, or what the point was.

And then, in March 2016, the first episode was released. Like a magic trick, the pieces came together. To say that IO Interactive’s Hitman is an episodic stealth game containing six levels is at once wholly descriptive and wildly inaccurate. The complete game, released on disc this week, takes Agent 47, the series’ bald, barcoded assassin, through six individual locations. No more, no less. That much is true. What becomes clear almost immediately, however, is that the nature of the game transforms those six levels into something expansive and remarkable, with a degree of replayability rarely seen before. This is not an easy game to explain, so let’s start here: Hitman is very, very good.

Facebook Twitter Pinterest The Marrakesh markets provide a suitably chaotic environment for a rather elegant 47 to go about his work. Photograph: Square Enix

On the face of it, this is a game about the interaction between two things: 47’s mechanical toolset, and the environment he finds himself in. The former appears to be very simple. 47 can crouch, take cover, vault over low objects. He can see a short distance through walls to identify the movements of his targets. He can throw small and large objects. He barely moves above a jog. It is a beautiful juxtaposition to his gliding menace, then, that the game’s most distinctive mechanic is a fundamentally silly one. Across the series, 47 has always been a master of disguise, capable of switching outfits on the fly to access off-limits areas. In Hitman, though, this is taken to a spectacular extreme as the bald assassin daisy-chains disguise after disguise as he moves through each of the vast environments.

If the mechanical toolset is simple and restrained, the six environments are the exact opposite. Each is a cross between an open-world and a Rube Goldberg machine, containing thousands of NPCs and nearly unlimited potential for inventive assassination.

The first mission, a fashion show in a repurposed Parisian museum, is a perfect demonstration of the game’s core design thesis: everything you would expect to be in such a location invariably is, and it’s all happening at once. On the ground floor, hundreds of guests in tuxedos and cocktail dresses mingle around the bar and the catwalk. Behind the scenes, stylists work industriously, stage crews guide models through darkened corridors. Downstairs, the kitchen hums as chefs and waiters work to keep the upstairs functioning. A security team have set up an outpost in the wine cellar.

Through this all, 47 moves like a shape-shifting ghost. At a slow walk, he acquires a waiter’s outfit from a side room, and slips down some side stairs into the kitchen. Nobody bats an eye. He corners a chef in a blind-spot and takes his outfit, doubling back into the kitchen and, safe in the plausibility of his disguise, poisons some soup. The disguise mechanic is brilliantly complicated by the presence of some NPCs that can see through them, presumably recognising that you’re not Karl the Stylist, and you’ll occasionally have to duck into cover or wait for one of these brighter specimens to pass ahead of you so as not to be spotted.

Facebook Twitter Pinterest The Sapienza mission takes 47 to a beautiful Italian town, where he quickly starts dealing with the tourist congestion problem. Photograph: Square Enix

Paris is followed by Sapienza, the game’s standout level, which takes in an entire Italian town. Then come the bustling streets and souks of turbulent Marrakesh, then an immaculate Thai hotel. The game’s final two levels, a terrorist compound and a high-tech Japanese hospital, push the players to perform increasingly demanding assassinations. The action of these missions move constantly around 47, and he shifts slightly to use their momentum for his purposes.

The reason for the scale of each of these environments becomes clear as the wider metagame emerges. Each level has main story targets, sure, but is also filled with an infinite number of player-made contracts, which ask 47 to assassinate a particular NPC, often while wearing a specific disguise or with a certain weapon. The crown jewels, though, are the developer’s own “Escalation” missions, in which the player is required to perform a hit five consecutive times, with new complications being added with each repetition. Kill the chef. Kill the chef and the waiter. Kill the chef and the waiter with a cavalry sabre. Kill the chef and the waiter with a cavalry sabre, but watch out for the deadly landmines placed around the museum. Now do it in 45 seconds. As IO added more escalations monthly, they became increasingly confident and absurd; they are beautiful practical jokes on the player, tense and funny and rewarding.

As 47 completes contracts and explores each level, he unlocks new tools and weapons to be used in any environment. Complete a hit effectively and you’ll be able to start with a disguise, or smuggle a large weapon into a box in the level. These spaces transform, both in actuality as you unlock things, and mentally as you build an increasingly coherent map of the spaces.

The game still changes and shifts monthly. New escalations are added, new contracts are featured. On four occasions, IO released “bonus missions” that radically transformed existing environments, filling the square in Sapienza with a film shoot or turning Marrakesh into a night-market. The game’s “Elusive Targets” showed up, too – unique contracts tied to an ever-descending real world clock that can only ever be attempted once, in situations that become almost unbearably tense. Fail them, and the opportunity is lost, never to return. While around 20 Elusive Targets have been and gone, IO plan to continue introducing them into the future.

Facebook Twitter Pinterest Agent 47 apparently shooting himself in the back of the head in Bangkok. Photograph: Square Enix

Throughout the course of this collected works, you will disguise yourself as a cyclist, a clown, as a po-faced doctor and a catwalk model. You will kill targets with a fire extinguisher, with a letter opener, with an exploding golf ball. At times you will flee from what feels like the entire level only to end up on the roof of an observatory looking over an Italian town. Sometimes, you will exit levels silently and beautifully and never be seen.

The beauty of the game is, you can play for many hours, but then see a friend try a mission and they’ll do something totally different. It might be a route you had never considered, or an interaction with a guard you didn’t know was possible. In this way the game unfolds like a puzzle box and, just when you think it has finished unfolding, it reveals something new.

Hitman is unquestionably the finest game in the series. It might be one of the best stealth games ever made.

Square Enix; PC/PS4(version tested)/Xbox One; £45; Pegi rating: 18+