There's a cold, callous core at the heart of Defcon. It's a game that thinks nothing of casually informing you of the death of millions of people, nor of advising you to take out your enemy's key cities and level them to the ground with nuclear warheads. And it does all this from its one cold, stark game screen.

But it's also an indie game hit, and for good reason. The easiest explanation of it, inevitably, points to the Matthew Broderick '80s film, WarGames. Remember that computer simulation of global thermonuclear war? That's been shoehorned into a fascinating strategy game.

The idea is as simple as attempting to destroy your enemy through all-out nuclear war, while stopping them from destroying you. Each side has key cities and installations, and up to six superpowers can compete at the same time on the same map.

The core of the game is very simple: you choose where to position your installations (which remain hidden from your enemies until they're picked up by radar, or you choose to use them), and then you have air, sea and structural resources to place. No big resource management here, though. You're given your tools and left to get on with the job, aided by a simple learning curve and a straightforward, to the point, interface.

There's the small matter of the clock, though. Defcon doesn't let you fire all your weapons straight from the off. Like a good game of chess or Risk, the trick is in the build-up, and thus the game starts at Defcon 5, which is a window of opportunity for you to sneak your subs into enemy waters, to position your silos, to place and utilise your radar stations and generally to plan your attack and defence.

As the clock ticks you move to Defcons 4, 3, 2 and ultimately 1, with the twist being that the game won't actually let you start using your munitions until at least Defcon 3, with more coming on-stream as you head to Defcons 2 and 1. You can speed up time if you like, but this is a game that likes to keep you waiting, and for all the right reasons.

Effectively then, the game breaks into a couple of stages. There's a few minutes of planning and strategising (finding the key installations you need to target, and working out how best to get them) and then there's the battle itself (where you switch your various craft and installations between defensive and attacking modes as required). There are so many different methodologies you can employ, and so many ways you can deploy your forces, that's it's one of the most taxing yet strangely straightforward strategy games we've played in years.

It really hits home, though, when you factor in the multiplayer mode. Playing against the computer-controlled opponents is one thing, but Defcon has been built from the ground up with online gaming in mind. The intensity of fighting one opponent is nothing compared to five other human-controlled players on the same map. The tension is palpable, aided by an outstanding and haunting musical score. And while you can sometimes complain that the game goes a little too slowly, it's nonetheless unlike anything we've played for ages, and all the better for it.

Defcon once again proves that if you're looking for genuinely fresh thinking in games, the shelves of PC World are not the place to find it. Available via the Steam download service or online, it comes with numerous game modes (which alter what you need to do to win a game), and a bargain price tag. Don't miss it.

One of the most interesting PC games we've seen for quite a while, even if the theme is pretty grisly.