What is it?
It's the revolutionary VW XL-1 - Top Gear's Innovation Of The Year, no less - reborn in less alien form, and on paper the most frugal small car on the planet.
The Twin-Up sees the XL-1's hybrid drivetrain - an 800cc two-cylinder diesel, alongside an electric motor and battery - transplanted into VW's dinky city car, an arrangement that promises a combined power output of 74bhp, along with official CO2 emissions of 27g/km, and consumption of 257mpg. Which means, with the Up's 7.2-gallon fuel tank a theoretical range of around 1900 miles on a tank. Obviously you'll never actually do that, but still...
Does it work?
We had but the briefest of runs in a very early prototype TwinUp, but it was enough to confirm that the XL-1's oily bits make perfect sense transplanted into an Up.
In electric-only mode, the TwinUp isn't quite so sprightly as the e-Golf we drove last week, nor indeed the all-electric (and appallingly named) eUp. But it's poky enough to do the traffic-light-to-traffic-light urban stuff that'll consume most of the Up's life.
In fact, the electric motor is slightly more powerful than that in the XL-1, to overcome the Up's extra weight and less piscine aerodynamics. It'll give you 50 miles before draining the TwinUp's 8.6kWh lithium-ion battery.
And if you need more shove, you simply call on the cheery diesel twin, which wakes up unobtrusively and emits the happiest of putters under heavy load. The two power sources combine seamlessly together to deliver more than enough go for suburban schlepping, and eliminate that pesky range anxiety suffered by owners of electric-only cars. Though 0-62mph takes a not-so-fast 15.7 seconds, more relevant is that the TwinUp will get from standstill to 37mph in under nine. Fast enough, just about.
Of course, you might well counter that, if you want a two-thousand-mile theoretical range, you don't really want a city car at all. But to us the TwinUp seems to represent the best of both worlds: a zero-emissions, cheap-to-keep plug-in EV for around town, but with the option of making it to Aberdeen in a single schlep when Auntie Morag comes down with her annual case of the willies.
With all those power sources on board, is there any room left for humans?
There is, for the TwinUp is most neatly packaged. The modest battery doesn't cut into bootspace or leg room, instead nestling right down by the rear axle. That said, this car is a few millimetres longer than the standard petrol Up, with the front bumper shoved forward to incorporate the hybrid's extra cooling elements.
As ever with VW, this is a proper job rather than a hurried conversion. Our favourite detail is the matching pair of filler caps on left and right: one for the diesel nozzle, the other for the electric flex.
Of course we'd love to have seen some of the XL-1's more outlandish touches - bullet-cam rear view mirrors, carbon chassis - make their way onto the Up, but that would rather undermine its billing as an affordable city-thing: the XL-1, remember, will cost you north of £100,000 to have on your driveway.
Even so, it's great to see VW's most advanced tech filtering so quickly down to cars within the reach of us mere mortals.
So should I buy one?
You can't. Yet. Officially VW says it hasn't decided whether to put the TwinUp into production, but insiders tell us we're likely to see the finished thing in 2016. If it can reach the road at a reasonable price - under, say, £15,000 or so - this thing could be a gamechanger. Watch this space.