Microsoft loves exclusives, but in Left 4 Dead's case it's really breaking new ground: If you want to play Crash Course on the PC, don't wait for me to convince you, just go ahead - it's free! If you want to play it on Xbox 360, you might want to hang back, because it's 560 Microsoft Points (£4.76 / 6.72). Apparently this is nothing to do with Valve, the people who made the content - it's because Microsoft doesn't want all the other premium DLC getting upset about being shown up. The fact we had to haul that explanation out of Valve suggests even the money men in Redmond find it a bit shameful.
They can at least drown their sorrows in the blood of a thousand zombies, because Crash Course is an intense little campaign that really kicks you around the houses for the half-hour it takes you to stagger through its two chapters. With the original four Left 4 Dead campaigns clocking in at around an hour each for a straight play-through, Versus games were lasting around twice that, so the idea here is to give the competitive side of the game's dedicated fans something that's just as demanding but over a shorter distance.
It also bridges a narrative gap - between climbing into the chopper atop Mercy Hospital in the first campaign and crawling onto a highway at the start of the second, Death Toll. We don't learn much more about what's going on in the world during Crash Course, but we do get some more quips from the crew (Zoey points out that climbing into a helicopter with a zombie pilot is poor survival logic), and judging by the writing scrawled on the wall of the midway safehouse we also discover that Valve thinks someone called Alison is a bit of a numpty.
With carte blanche to fill the intervening period, it's perhaps a little disappointing to start off by running around the same dark alleys and warehouse interiors that we've raced through dozens of times before. Articulated lorries and wire-mesh fences funnel you through buildings and across a few streets before you drop down to a riverside and circle up to a bridge; then following a crescendo moment involving a Howitzer you cross the water, do a safehouse transition and fight your way to a truck depot where you need to power up a generator while fighting off the horde for a finale. (The Howitzer and generator bits form the basis of two new Survival mode sections.)
That initial disappointment seeps away though as you start to appreciate the devilish way Valve leads you into unavoidable traps and how even seemingly incidental touches - like shrubbery covering the approach to a raised highway - are calculated to make you work harder for survival. The area surrounding the Howitzer crescendo is particularly well appointed - having dropped down onto a road by a river, my first group of survivors was quickly assaulted by the horde, and my attempt to avoid them by standing on a car was upset somewhat when a Tank popped up and grabbed the vantage point out from under me. The AI director is on his usual form, too, barely touching us the next time we dropped down - although he did throw in a Witch.