More than a mere palette swap, this is a sequel packed with improvements and impressive technology, although many of the core values that make a Call of Duty game what it is have been retained; aiming down the barrel of the myriad guns on offer feels just as good, if not better than before, and the on-screen indicators that warn of nearby grenades instil a familar sense of panic. Also unchanged is the scripted nature of the single-player Campaign which forces the player towards specified checkpoints and targets to forward the story. Many of the missions, however, offer quite a bit of freedom with no real "correct" path to follow, leaving it up to the player to find a strategy that works for them. As a result, much of COD4 doesn't feel quite as restrictive as its predecessors.

The actual story too, is a big improvement and bang up-to-date, containing many themes and issues that could easily be lifted from today's headlines. It concerns a tale of Russian ultra-nationalists, who, with the help of a Middle-Eastern terrorist group, are threatening to use stolen nuclear weapons against the West in an act of revenge for "corrupting" their society.

As in the previous games, COD4 features multiple playable characters. In this case, the main protaganists are a rookie S.A.S. officer and a US Marine with the action switching between them at various stages of the campaign, although for the most part, the game is viewed from an S.A.S. perspective. The narrative is tighter and more focused than before, and, with the exception of a superb post-Chernobyl flashback section, is one large continuous story arc. There were instances in COD3 that often left you thinking "who the hell is this guy and why am I controlling him?", but in Modern Warfare, there's a real sense of progression and the characters are well fleshed-out and believable thanks to some excellent storytelling and voice work. You really get to know, and care about them, which makes some of the plot twists along the way all the more shocking.

A good story means little without good play mechanics, especially in a FPS, but this is also an area in which COD4 excels since mission variety is very good and each of the weapons available feels (and sounds) entirely convincing. Whether picking off distant enemies with sneaky headshots from a sniper rifle, or emptying an entire clip into a foe up close, it just feels perfect, and hugely satisfying.

In an innovative move, the attention to detail is also extended to the environment; taking cover behind soft materials like wood or plaster walls is no longer a guarantee of safety as some weapons will penetrate those surfaces quite easily, and although a bullet coming through a wall won't have quite as much impact, it will still be enough to kill if low on health. This is an important aspect of the game and becomes central to the player's thinking because finding cover is an absolute necessity in COD4, and here's a one word reason why - "intense".

Never before has a shooter felt quite so intense and visceral as COD4. At times the amount of onscreen action is almost overwhelming due to the sheer number of enemies, and in some parts of the game they will just keep on respawning untill the next checkpoint is triggered. This makes for some fairly nasty difficulty spikes when playing on Veteran level, where each enemy seemingly acquires superhuman eyesight and reflexes. Thankfully, though, the pause between dying and having another attempt is only a couple of seconds - just long enough to read one of the famous quotes from a historical figure that follows each death.

On normal difficulty, the game is still an exciting and hugely entertaining experience, only less lethal and way more forgiving. In fact, it's probably a shade too easy on normal, so we would recommend playing on "Hardened" at least, just to up the tension level and stretch the Campaign's length by a few hours. Completing the game unlocks the Arcade mode, which is a score-based version of the campaign that employs a time limit, bonus multipliers and restricts the player to a set number of lives.

It's a nice little extra, but the real reason to come back to COD4 once it's finished is the multiplayer mode, which is frankly staggering and arguably the best online FPS experience available right now. Unlike most games, the multiplayer portion of COD4 was developed alongside the SP campaign from day one, and it shows.

A variety of team and solo modes are available ranging from standard game-types such as deathmatch and capture the flag to more tactical objective-based games where bombs must be planted and defused to earn victory. All of them are excellent fun and a party system is in place for friends to team up and play them together, and with 16 maps available, there's plenty of variety on offer.

Each of them is extremely well-designed and throws up different playing experiences due to their size and configuration. Huge sprawling epics, such as Overgrown and Pipeline feature some lovely long grass and elevated positions ideal for snipers to work from, whilst smaller, more confined maps such as Wet Work or Vacant have very few hiding places and result in fast-paced carnage. Falling somewhere in-between are a selection of street setups for urban-style warfare.

The great maps are a big factor in why the game is so much fun, but the real depth to the multiplayer side is down to the vast amount of options and customisation on offer.

Whatever your playing-style, there's a large selection of weapons, equipment and "perks" available to tailor your online persona to your needs. Aggressive players who like to get right into the action can tool-up with grenades, shotguns and powerful machine guns for Rambo-style action, whilst those who like to adopt a more calculated, stealthy approach can also equip themselves accordingly with long-range weapons, zoom scopes and silencers. Another layer of customisation comes in the form of the aforementioned perks. Up to three can be selected, and these range from additional equipment (Claymores, radar jammers etc) to special abilities such as deeper bullet penetration, increased health and so on.

There are also three in-game perks which can be earned during gameplay; the UAV, which is a radar sweep that highlights enemy positions on the map, an Air Strike, where a squadron of jets blitz a specified area with bombs, and most potent of all, a helicopter can be called-in to circle the map and hunt for enemies. Each of these becomes available by achieving a three, five or seven kill streak respectively and earning one can make a huge difference to the outcome of a match.

Additional weapons and perks are unlocked by earning enough XP to move up the ranks all the way to Commander (Lv55). XP points can be gained in a number of ways, mainly from kills, assists and finishing matches, but large amounts are also awarded for completing the myriad of Challenges listed in the Barracks. These are like mini achievements and range from the basic (150 kills with weapon X) to something more grin-inducing like stabbing someone in the back, or killing an enemy, then picking up their gun and using it to kill them again.

Upon reaching the rank of Commander, the choice to enter Prestige mode becomes available, which basically resets the rank and unlocked gear back to zero so you can do it all over again - the only real reward being a little insignia to signify your Prestige status. The progressive ranking-up and unlocking of new goodies is a big part of the game's appeal though, and frighteningly addictive, so it's a choice many players will find easy to make.

Quite simply, Call of Duty 4 is a stunning game and quite difficult to fault. It looks, sounds and plays brilliantly and offers an unrivaled online experience. If we're to nit-pick, then the respawning enemies found in some parts of the Campaign can be frustrating and feel a bit old-fashioned compared to how advanced the rest of the game is. Really though, that's just about it. Go and buy it. Now.