You have to take Driver: San Francisco with a pinch of salt, or - if you're less forgiving of the supernatural than I - a generous handful. The narrative underpinning the innovative new Shift mechanic is a little...out there. As Tanner cruises the sun-drenched streets of San Francisco, he can choose to ditch his iconic Dodge Challenger in favour of any other car on the roads. Not by jumping out and jacking it GTA-style, though. Oh no. He can project his consciousness into the body of another driver, becoming that person. Yeah. But it's fine, he doesn't actually possess superhuman powers, because he's dreaming the whole thing!

In real life, Tanner is tucked up in a hospital bed, only breathing thanks to a little plastic tube. After pursuing escaped convict (and arch nemesis) Jericho, the undercover cop gets tangled up in a rather nasty car accident. Jericho ploughs a police security truck into his car, in fact. He thinks he made it out with little more than a bump on the head, but sadly this isn't the case. In reality he was rushed to hospital in an ambulance and has remained unconscious since. The majority of the fifth Driver game plays out in Tanner's coma-induced brain.

It's a hard sell, but the innovation justifies it. Just. Shifting from car to car is something the genre hasn't seen before, allowing for several entertaining additions to the established Driver formula. Police on your tail? No problem: simply jump into another car to lose the heat. Caught up in rush hour traffic? Then use Shift to zip to the other side of town in less time than it takes to change gear. Planning on filming an epic car chase in Director mode? Why not steer oncoming traffic in front of your cameras, taking out cop cars and creating dramatic crashes in the process.

What's important to take note of here is that you're not just an undercover cop: you're a taxi driver, a medic, a cameraman, a drug dealer, a kid having his first driving lesson - you're anybody in the city that might find themselves behind the wheel of an auto-mobile. What are known as City Missions are built around this fact: side-quests that you can jump out of the main storyline to play. In fact, the game forces you to complete a certain number of these before it'll let you continue the story, which could quickly become irksome if variation is lacking. The few I played, however, were all interesting enough.

Take possession of a police car, for example, and you can engage criminals in frantic pursuits through the city. Shift into the mind of a rich business man, and you might find yourself test driving the latest sports car, scaring the dealer senseless by driving like a lunatic. My favourite of the few I played cast me in the role of a teenager during a driving lesson. Using drifts, jumps and near-misses, the idea is to increase the heart rate of the lad's irritating instructor. If Tanner doesn't like his passenger, he won't hesitate in using his talents behind the wheel to scare the wits out of them. You'll always return to Tanner's yellow Challenger to advance the main storyline, but you're rarely in the same car for very long.

Driver: San Francisco was originally scheduled for release at the end of last year, but Reflections has squeezed an extra year out of Ubisoft to ensure the game is up to snuff. This was predominantly for technical reasons. From the outset, the team has strived to deliver 60 fps - a tall order for an open world driving game of such scale. Having played the first hour of the game, however, it's clear the game is running pretty darn close to that figure. With detailed character models, stunning urban vistas and a silky frame rate tying it all together, the extra year in development appears to have been well spent.

Many will struggle to get over the whole 'you're in coma' thing, but there's no denying the Shift mechanic at the heart of the game is an interesting one. The game is still a few months off and I find myself in a position I seldom find myself: I have no idea what critics are going to make of it. Until I see how the coma situation pans out and get stuck into the Director mode (which was easily the most enjoyable part of Driver 3), I'm going to have to remain firmly on the fence with this one.

Driver: San Francisco will be released for Xbox 360, PlayStation 3 and PC September 2nd.