A leap from the plinth lands on that familiar bed of sloping sand. The floor has a bit of texture to it, of sliding grit and twinkling silicate. The world is a big channel around us, tubing down, down, down through the clouds. In the distance is the familiar shadow of the mountain, but before that paths to choose at speed, glowing lenses of light to pass through, and at the end, as the rushing of wind becomes overwhelming, two giant doors slowly open, just because we have arrived.

This is Sky: Children of the Light, the latest game from Thatgamecompany. And it is also a great moment from Journey, the studio's dreamy, religiously-inclined mega-hit from a few years back, though it feels like a whole era of video game history ago. The difference with Sky is not just that the moth-brown traveller has traded their scarf for a cape, or can be decked out with different hairstyles. The difference is that this moment unfolded for me on the 14C, a bus that rattles along the Sussex coastline before swinging inland to pass the Royal Sussex Hospital. Thatgamecompany's previous games delighted in filling very large screens in the living room. This one twinkles like a private grotto in your hands. It's an adventure that plays out on your phone.

What kind of adventure? On paper, it sounds very different: smartphone-based, free-to-play, if not quite an MMO then at least a live game that changes over time and allows you to mingle with hundreds of other players. In reality, it is business as usual for Thatgamecompany, at least over the course of the three or four hours I have put in so far - time which has allowed me, I gather, to get through about three-quarters of what counts as the story stuff that's currently in the game. Business as usual for these guys means that the game is extremely beautiful on my iPhone screen, rounded hills of shimmering sand or waving grass, pads of cloud in the blue sky, a numinous mountain with a light on in the distance. The free-to-play stuff seems to be generous. There are two currencies, as far as I can tell, and a season pass, but a lot of what you can buy is cosmetic stuff and the main content has been entirely free so far. A good marker of how generous this game is, is that, after those three or four hours, I still don't really know what the currencies do, and I haven't found myself brought up sharp by a velvet rope. As for the live game, there is chatter of events to come, and there's a quest giver at the starting hub, and I mingle with people in classic Journey style, communicating through emotes, bowing, nodding, singing, holding hands.

Even so, it's taken me a while to understand exactly what Sky is, but I think I'm getting there. This is a puzzley, platformy, explorationy game with that familiar nondenominational religious vibe and a story that hinges on candles and constellations. I move through various beautiful, often sparse environments, from deserts to buried forests and huge temples. I light candles, follow ancient spirits from one point to the next, open doors (often with the help of other players), and meet a series of solemn giants. Everything is wordless, the air is always charged with expectancy, and nothing outstays its welcome.

Controls are simple, and while there seems to be a bit of a grump about them online, I'm a fan. The left side of the screen allows you to move your character. The right side allows you to move the camera. There's a button for emotes, which you steadily unlock as you move through the game, and there's another button for jumping - or flying.

Oh yes, you can fly in Sky. It's another lift from Journey - there's that same sense of billowing up and catching the thick, grainy thermals of the wind. But now you can really soar, carving soft furrows through clouds, racing up the sides of temples to hop, softly, onto a plinth, moving through a valley in the company of bleached manta rays, ringing the bells on every temple turret as you pass. The freedom, as is always the case with Thatgamecompany, is illusory. (Another familiar element from Journey is the way the camera will gently reposition itself as you climb a dune, a gentle hand on your elbow, quietly guiding you.) Instead of freedom, what you get is moment after moment of choreographed spectacle. Epiphany after epiphany.

And this may be a problem with this beautiful, rather generous game, that offers so much at present and seems, at least, to ask so little in return. It is an epiphany slot machine. Every few minutes the music swells, the light changes, the temples rise up and the world is transformed by my character's presence. At its best it can change the tenor of a bus ride, as with my journey down that valley, skimming the sand, sliding between obstacles. At worst, it can be like eating your way through the world's largest marshmallow.

For me, the game that truly haunts Sky so far - and I appreciate that I am so early in that any kind of verdict would be largely unfair - isn't Journey. It's Rime. Rime used the same dreamy, quasi-religious setting, the same vast architecture and playful puzzling. But it built it around a core that was, to borrow the words of that guy who really likes decorative gourds, extremely fucking real. Sky, by comparison, can feel a little like a richly talented team laying on effects because it's what they've always done. At times, the developers at Thatgamecompany can feel like prisoners of their own good taste.