Rez is a rail shooter in which the player takes control of an onscreen avatar traveling along a predetermined path through the computer network. The player does not control the overall path, only the avatar’s position on the screen. The player targets foes by holding a « lock-on » button while moving an aiming reticule over up to 8 enemies. Once the « lock-on » button is released, the avatar fires shots that home in on each target. Failure to hit an enemy or projectile in time may cause a collision, which reduces the player’s current evolution level by one and changes the avatar’s form. The game is over if the avatar is hit while at its lowest possible level. At higher evolution levels, the avatar appears as a humanoid figure, while it appears as a pulsating sphere at the lowest level.
Occasionally enemies drop one of two power-up items when destroyed. The power-ups either enhance the player’s « evolution bar » by one or three points, or they enable the player to trigger an « Overdrive », which releases a continuous shower of shots at all enemies on the screen for a short period of time. In some game modes, score bonus items also appear periodically.
The game consists of five main areas. The first four are divided into ten sub-sections and conclude with a boss battle. The final area contains a larger number of sections and a boss rush, in which the player must fight variations of the bosses from the first four areas. The player then goes on to the network’s core to restart Eden in a final boss battle.