The dazzling debut effort from Hemisphere Games, Osmos is a short, sweet and suitably soothing downloadable title which more than lives up to the independent Canadian developer's self-stated mantra: it is, truly, an experience which will stimulate both sides of your brain. There's so much more to its physics-based gameplay than it seems, and although it ends a little before it should and there's a shortage of visual variety, Osmos should be a shoe-in for consideration as 2009's casual game of the year.

The handful of minds behind Osmos cast you as a mote, a tiny particle of organic matter in the vast biospheres that act as levels. A few significantly less expansive introductory areas serve to teach you the basic principles of life as a small, circular blob, which is to say that for every action you take, there is an equal and opposite reaction. This deceptively simple Newtonian dogma informs every facet of gameplay in Osmos, from movement to absorption to the more complex requirements of scenarios towards the end of the experience. To gain momentum, for instance, you must eject matter away from the direction you mean to travel, but for each globule of yourself you expend to achieve propulsion, your mote becomes proportionately smaller. When your mote is but a microscopic molecule, as at the beginning of most every biosphere, it's a sprightly, responsive thing to steer - suitably easy to control once you get your head around the Katamari-esque movement mechanic; the larger it gets, however, the more sluggishly it turns and the more propulsion you'll require to maneuver its augmented mass, thus the more of yourself you must sacrifice in order to achieve your objectives.

The majority of Osmos' levels call for you to become the biggest mote of all. To do this, you must absorb - simply by moving into their proximity - those motes in the biospheres that are smaller than your own. Close encounters with bigger motes, conversely, are to be avoided, as they will absorb you. As you grow, however, you'll be able to absorb progressively larger masses, until you are, indeed, the biggest mote of all. On paper, the act sounds perhaps a little more complicated than it is, but Osmos is in any case intuitive enough that you'll have the basics down within a few, floundering minutes.

There's more, of course, to the experience than the simple but addictive balancing act of absorbing more mass than you expel. Once you've done away with the tutorial levels, three differentiated modes of play emerge, of which there are around ten levels each. Ambient has you growing ever-larger and absorbing motes on an exponential scale, and feels like the most natural extension of the mechanics introduced thus far, including several so-called impasses, maze-like puzzle stages which are perhaps the highlight of the game. Force, meanwhile, introduces less predictable biospheres, such as those where you orbit a small sun, and self-explanatory creatures called attractors and repulsors which, taken together, change up the formula of Osmos considerably. Survival takes that evolution another step, introducing competition in the form of more intelligent motes than those that simply float around according to their own simulated physics, as well as nemocytes and biophobes, enemies which will come at you just as you attempt to grow large enough to absorb them.

For an experience that places such emphasis on its zen-like ambience, the difficultly curve in the later stages of Force and Survival becomes so steep as to be positively vertiginous. The hypnotic pace that Osmos sets so perfectly and extends in the Ambient mode falls somewhat by the wayside when to survive, to succeed, your wits must be sharp and your reaction-time almost nothing. A little variety goes a long way to prolonging the longevity of a casual game such as this, and while the option to randomise and replay any level sees to that requirement, such tense, twitchy play as there is to be had in Force and Survival mode sadly proves counter-productive in terms of the overall tone of the experience; there's simply nothing relaxing about being absorbed again and again by spiteful motes whole singular purpose is to make your passive existence a nightmare of frustration and exasperation, and these feelings undermine the blissful tranquility which the more soothing sections of Osmos evokes so successfully. Better by all accounts to take your time dodging dark matter and whittling away at massive motes with an eye to eating them sometime soon on an impasse arena; better, surely, to become the biggest mote of all as calm waves of minimalist electronica wash over the experience and lush, organic visuals lull you into the dreamlike atmosphere.

It's a disappointment that there isn't more to the Ambient mode, that precious resources which could have been spent building it out more thoroughly have instead been channeled into ill-advised modes of competitive play which only serve to detract from the vastly more rewarding experience at the slow-beating heart of Osmos. What Hemisphere Games' debut does well, however, it does so without equal. Its more relaxing aspects - and there's enough of those to make this downloadable gem well worth the impulse price of admission - expand brilliantly upon the themes Jenova Chen and thatgamecompany have worked to introduce to the gaming landscape. Despite its problems, then, Osmos is an ambient Katamari Damacy for the flOw generation, addictive and emotional and singularly satisfying.

Reviewed by Niall Rough for AceGamez (All Rights Reserved).