This must rank fairly high on the list of Things That Shouldn't Work (But Do) alongside such crazy rulebreaking anomalies as jumbo jets, Noel Edmonds and peanut butter and marmalade on toast.
A DS version of a game that's successful pretty much exclusively because it comes packaged with a massive plastic guitar and makes you feel like a rock god. That's going to be completely rubbish and depressing on a handheld, isn't it?
But the DS is no ordinary handheld and Guitar Hero On Tour is not the cash-in many observers had expected. Like its console big brother, it comes with a large and expensive peripheral that, once you get used to it, makes perfect sense.
The guitar grip accessory is chunky piece of plastic, about the size of a fist, that wraps around the back of the DS and plugs into the GBA slot on the bottom. Holding the DS sideways in book format (same as Brain Training and Hotel Dusk) you slip your hand through the padded strap and let your fingers rest on the four coloured buttons.
Plug and play
Yes, it looks nothing like a guitar and is not entirely comfortable to use at first, but after short period of acclimatisation it begins to make sense.
It doesn't add so much weight to the DS that it becomes a pain to hold, but it's solid enough to justify the £45 asking price.
Other DS accessories such as the slide controller from Mag Kid, the camera from Face Training and the paddle from Arkanoid, haven't been so expensive but we'd have to say the guitar grip is the most substantial of them all.
It also comes with a special stylus shaped like a plectrum, which slots onto the top when you're not using it. Whip it out and you're ready to rock, strumming the guitar strings on the touch screen.
The touch screen takes the place of the strum bar in the console versions - you don't actually hit the individual strings. Status displays such as your combo meter and star power indicator are also shown here.
When you've got the knack of holding the four coloured buttons and swishing the stylus at the same time, Guitar Hero On Tour plays very much like the console versions.
To play short notes you do a stacatto flick on the screen in any direction. To play a longer note you move the stylus-plectrum on the screen and keep it pressed down for the duration.
You can still do the up-down strumming favoured by expert players of the console games, although left-right might prove to be more a comfortable direction, depending on how flexible (or knackered) your wrists are.
One of the chief differences is that in having only four buttons instead of the usual five, there's a whole difficulty level missing. However, it's better suited to handheld play without the screen-filling nightmare that is Insane mode - you'd have arthritic claws for fingers after half an hour of that on DS.
Since the guitar grip's strap and buttons effectively fix your fingers in place, there's no sliding around the frets. If you found the full version a bit tricky, even with four buttons, you might have better luck with it on DS.
Hitting a successful streak of notes will fill your star power bar, which you activate not by moving the neck of the guitar (there are no motion sensors in the guitar grip and, erm, it doesn't really have a neck anyway) but by shouting into the microphone.
This is one DS game you're not going to want to play on the train. You can shout 'star power' or 'rock out' or just do your very best rock scream.