The opening track by Keiichi Sugiyama, "Buggie Running Beeps", opens in much the same fashion as Vib Ribbons' first song, "Polaroid"; both start at a "walking pace", before the BPM slowly creeps up and the track properly kicks in, giving the player a minute or two to grasp what is required of them - holding X to begin, using the analogue stick to scan the approaching enemies, then with the release of the button, sounds are added by the player to the track as an attack clears the field of incoming viruses. While some require only one lock-on to take down, many of the larger viruses need to be scanned a few times. This all takes place in time with the music playing on any level and the sounds produced by attacks never really clashing with the main themes. This is a true on-rails shooter too, the player's field of vision and where they travel are limited to what the game permits and while this may sound restrictive, it allows the developer to take the player on the journey it wants us to see. On paper it sounds very simple and basic and while this is true up to a point, it's something that has to be seen, heard and played to be really understood.

The avatar in Rez has the ability to evolve to a higher state, or devolve to a final lower form, reliant on the player's ability to scan as many of the enemies, incoming missiles and the desirable support items as an increasingly numb thumb will allow - much as Vibri does in Vib Ribbon, evolving or devolving in relation to the player's reactions. Essential to progression and completion are the support items which in the regular game come in red and blue varieties, with a third green version added for the Score Attack. Blue items come in two sizes and are needed to fill a progress gauge (made up of eight bars) and once that is full the avatar will evolve into the next form. The normal one will fill one bar in the gauge and another larger version fills three. This also brings a new attack pattern and new sounds to the mix.

Red items are Overdrives. Releasing an Overdrive covers the music with a progressive wash, a crescendo that is as soothing to listen to as it is to watch. For a meagre few seconds the game takes control as the current form's attack is played out across the screen. The Overdrives should be hoarded in the same way as bullets are in a survival horror game, only to be used when facing tremendous odds or the end-of-area bosses. A screen full of oncoming enemies can be so overwhelming sometimes that it can be all too easy to miss one tiny missile. If players are hit just once the avatar devolves, take as hit in the lowest form and it is game over. This is gaming on a knife edge.

Although there is a story to be told it's not overly complicated. A programme called Project-K (the original name for the game) was created to help solve the world's myriad problems, but the A.I at the centre, Eden, has developed beyond her original programming, including a conscience it would seem. The aim is to fight through the viruses and firewalls (the bosses) to reach the final fifth area. Simple maybe, but it's enough to carry the game through to its conclusion.

There are many visual highlights in the game, with the colourful vector-style graphics having image and colour-themed levels, each with unique bosses. Before reaching Eden there are four end-of-area bosses to deal with - Earth, Mars, Venus and Uranus, with Coldcut taking over music duties for each. All are distinctive, but it is perhaps Uranus that stands out. As the player comes to the end of the area, the beats and bass drop out of the main theme and as the player battles through to level ten of the area, it's like a calm before the storm. The background turns from black to white, throwing the colours into sharp relief and a tunnel appears on the left of the screen. Without warning, the drums and bass kick in with venom, guitar samples give the game a drive and urgency and the level boss appears - blocks in the shape of a man running further into the programme, twisting and turning down a labyrinth of tunnels, as much in an effort to escape as in an effort to pull the player further into its world. Within this maelstrom of shapes a saucer must be picked out and it takes a fine eye to do so, disguising itself in many forms.

What many players didn't at first realise was that the game adapted itself to how well the game was played. Upon reaching an end-of-area boss, a steam of data appears. Apparently about nothing in particular, it contains information on one of the three levels of difficulty of the boss the player is about to face - Mega, Giga or Tera. The percentage of enemies shot down (a number only revealed after the level is finished) will in turn affect the boss finally faced - 98% will produce a Tera boss, the hardest of them all. There are major differences between these difficulties and it allowed the players who wanted to finish levels the opportunity to do so, while also providing more of a challenge for those who become more proficient - the twitch-style nature of the gameplay is something that can be mastered with time. Reaching 100% for all the achievements can seem daunting at first, but can be accomplished.

The main game is actually very short, playable the first time in around an hour, experienced players can finish it off in less than half that time. There are no side missions to distract, no secrets to discover or coins to collect and only five levels to complete. Its simplicity is its saving grace however, for that Holy Grail of gaming replayability is here. Finish each level and the Score Attack version of that level will be unlocked. Substantially different enemies appear here. Ever tried to swat a house fly? That may give some idea of what a few of the newer adversaries are like to pin down. They dance and spasm out of harm's way, seemingly anticipating the player's every move. Bigger enemies spew forth swarms of smaller foes to be dealt with. This is no mere retread of the main game, but an expansion on themes that instead increases the difficulty. No bad thing, as battling against tremendous odds and a screen filled with enemies is a pleasure rather than a chore. Also, the green support item appears in these levels to increase the final score.

It is from achieving top rankings in the Score Attack that more of these begin to open up - Direct Assault (play the five levels back to back); the Lost Areas, featuring new individual areas from "Forgotten Realms of the Network Past" and brand new tracks to go with them; Trancemission, another version of the game, suffused in light and sound and finally Boss Rush, where all the final bosses can be fought back to back. There are even options later to change the forms players can begin with and the lasers they use. There is much to be discovered if players put the time in. But beyond all the statistics and percentages, Rez is a fun game to play. Seemingly basic and lacking depth at first, it reveals itself over time to be a game of many games.

If it can be said that a year is a long time in videogames, then five years may be a lifetime. It's now been five years since Rez was released. While not a huge commercial hit initially, the game is held in very high regard today due largely to word of mouth.

It can be all too easy to bandy about words like "classic", "essential" and "influential" without being truly aware of their impact and overuse can dull them even further. Many games receive such accolades and few actually deserve them. The fact that the game can stimulate such debate, wonder and remain, even now, huge fun to play is something for Tetsuya Mizuguchi and his very talented team to be rightly proud of. This is proof that a good idea, well-executed, can stand the test of time.