This is one of those moments that make a motoring journalist's life. A car shaped for the future, first revealed as a radical 1997 Frankfurt motor show concept, is about to take to the road in a form ready for you, me or anyone to buy and use. I am going to drive it, and future is about to become present.

Seeing Audi's radical A2 under a motor show's bright lights is one thing, but how will it look out in the open, on real roads, interspersed among more prosaic fellow cars? (Terrific, actually.) Will people point and stare? (They do.)

And how will it feel to drive? That other future-radical car, the Citroen DS, was always best admired when stationary. On the move, its grumbly old engine sat oddly with the space-age shape and Dan Dare detailing, breaking the spell. The A2 shares its engines with Volkswagen's Polo and Lupo, so these engines are a point of contact with the known automotive universe. Might they, too, burst the bubble?

With radical looks goes radical construction. The A2's structure is an aluminium skeleton of castings and extrusions clothed in lightweight panels. It weighs 40 per cent less than if it were made of steel, to the great benefit of performance and fuel efficiency, but costs two-and-a-half times as much to manufacture. Audi has used this technology before, in the large, expensive and exclusive A8, but this is the first time a low-cost, mass- produced, profit-generating car has been formed in aluminium.

If the design language is unmistakably millennial Audi - the hefty wheel arches, the headlight shape, the domed roof - the size and proportions are more Mercedes A-class. The front wings are bendy plastic, the wheel arches are separate, easily-replaced mouldings, the roof has four stiffening ribs and the tailgate and rear window are cut far into it. There is no rear wiper, but the window has a "hydrophobic" coating. Do not let your dog lick it.

Inside, you will find a style and substance to shame both the A- class and Audi's own A3. Nearly every surface is soft to the touch, unless it is the aluminium of the gearlever stem, the glovebox catch or the dial surrounds. The design theme is bold, horizontal lines. You sit quite high, but not MPV-high; ahead of you is a twin-armed, single-bladed, pantograph windscreen wiper, behind you are just two foldable and removable rear seats. This is surprising, and bodes ill for the A2's role as a family transport capsule, but a three-seat row is optional. This lacks a full three-point belt for the centre occupant, which will come in due course.

Adults may find a lack of rear legroom, but children will fit comfortably. If they are small, you can take advantage of their short legs and specify the optional floor boxes which fit in the deep rear footwells. This extra storage space could be useful, because the boot is not large despite a small-diameter spare wheel, which needs to be inflated before use (an electric pump is supplied). Another option is the "open sky" full-length glass roof, complete with a sunscreen of aluminium mesh. It looks great, but cannot do much for the A2's lightweight credentials.

What appears to be the radiator grille is a plastic flap which flips down to reveal hinge-out orifices for oil and washer fluid, plus the dipstick. The bonnet does not hinge open, but is unclipped for servicing. Apparently, today's drivers dislike opening a bonnet and so neglect these essential checks. Now they need never be confronted with signs of mechanical mystery, and will not know what the A2's two 1.4-litre, 75bhp engine options - a four-cylinder petrol and a three-cylinder TDI turbodiesel - look like.

The TDI is the better choice for the A2-shaped, car-reinvention experience. Its deep thrum is a suitably off-beat soundtrack, and it delivers a torrent of torque absent from the quieter, smoother, freer- revving but blander petrol engine. The diesel A2 is much more economical, too, even though it weighs 990kg instead of 895. That extra weight is nearly all over the front wheels, so it is the petrol A2 that better delivers the agility, precision and lightness of dynamic touch that the lightweight construction promises.

But with quick, sharp steering and nearly lean-free cornering, both A2s are fun to drive. Do not be tempted by the optional fat wheels and ultra- low-profile tyres, though, because they destroy the suppleness you get with the standard footwear. You will need to develop a supple neck, too, because those thick, curved windscreen pillars block the view badly. On one twisty stretch of road, a preceding Renault 5 disappeared entirely.

The petrol A2 arrives here in September, and the pricier TDI follows in January. Should you buy one? Fuel consumption apart, it offers little practical advantage over a regular five-door hatchback. But that is not the point. Drive an A2, and you drive a piece of the future.