Children in particular will relate to the design principles behind the extraordinary, shape-shifting Citroën C3 Pluriel. As I crouched over the tailgate with the manual open, thigh-deep in detached pieces of the car's anatomy, a boy of four asked me: "Are you transforming it?"

And that's exactly what the Pluriel is: it's an adult Transformer or Go-bot. You pull its backside over its head and - hey presto - it's a different creature altogether. Or a fairly different creature, anyway. Because the Pluriel doesn't quite extend to changing from a car into some kind of laser-toting dinosaur, the four-year-old boy looked mildly underwhelmed when I'd finished. Nevertheless, it does go from saloon to cabriolet, and then on to an utterly roofless, spider-style pick-up configuration, which is a pretty good trick if you can manage it.

And the Pluriel does manage it - and easily, as long as you can deal with the bits that get left behind during the transforming. But let's start from the beginning. Untouched and straight out of the box, your Pluriel is essentially a soft-top version of the chucklesome C3, the snub-nosed, highly covetable supermini that Citroën launched last year to much acclaim and big sales figures.

Distantly reminiscent of the legendary convertible Citroën 2CV, the Pluriel's canvas top covers the roof from front to back, but it folds electronically at the twist of a knob, sparing you any of the tugging and buttoning that made re-covering an old 2CV the total body workout it once was.

So, you can drive your Pluriel wide open to the sky with the roof right back - just as long as you don't mind the wind thrumming in your ears at a number of variously tolerable frequencies. And as long as you don't mind not being able to see anything at all out of your rear-view mirror because, fully retracted, the roof gathers on the back window.

But here is where the Pluriel starts to get cunning. The back end is split and comprises a pop-up window, hinged at the top, and a drop-down tailgate. The whole of the window section can be pulled slightly adrift of its moorings and pivoted magically into the boot, which turns out to have a false bottom, like a spy's suitcase. Replace the false bottom, slam the tailgate and not even a customs officer with a sniffer dog is going to be able to find that back window of yours.

You then tidy up the rear end by snapping on the moulded plastic boot cover and side-flaps which, up until now, you have been carrying around in the boot's secret compartment where nobody knew anything about them. And because all the side windows go down, you can enjoy a completely glassless driving experience (windscreen and front quarter-lights aside), leaving the wind no option but to find someone else's car to thrum in. At this point, though, you may be tempted to go completely native and take off the car's roof beams. On most models, you'd need an arc-welder and a steady hand. Here you do some simple uncoupling at each end and lift. (Reassuringly, the beams go back just as straightforwardly.) At this point, one realises that one has taken apart half a car with one's bare hands - surely a landmark moment in the history of clip-together parts.

One snag: you are left staring down at two fairly heavy 6ft branches of plastic, like fake dinosaur bones. And because there is no secret compartment in the world large enough to conceal these, you have no choice but to leave them behind.

Courageously, Citroën offers, as an optional extra, a tall, thin, T-shaped metal stand on which you can vertically hang your roof beams. This is by some degree the most remarkable car accessory I have ever seen. Thus arrayed, in the right sitting room, your suspended roof beams could probably pass for a sound sculpture or a really avant-garde pair of hi-fi speakers. But one would tentatively suggest that the Pluriel was not the car for you if you lack garage space. At the very least you'll need a big hallway.

You will also need an accurate long-range weather forecast, because if it rains while your roof beams are at home, you will have nothing to pin your Pluriel's roof on. The upholstery is waterproof and Citroën do a natty full-length pac-a-mac to wrap your Pluriel in when it's stationary. But you will struggle to drive the car from underneath one of those - certainly without arousing the suspicions of the police. So your options in a downpour are: wait or get wet.

Still, how often does it rain these days? I gaily cast aside my roof beams in Norfolk last week and spent many pleasurable hours skipping up and down country lanes with the wind in my lap and my window in the boot. It was like finding oneself in a bumper car on the open road, except the Pluriel corners more accurately, works harder under acceleration and can carry two adults and two children in comfort. True, it's not a tooled-up dinosaur. But it's more fun, if that's possible.