The goal of Reaction is to destroy as many targets as possible with as few shots as possible. When a target is hit, it explodes and sends out additional projectiles that can strike other targets. These additional projectiles do not use up any the player's shots. The longer a chain of consecutive hits is, the more projectiles that will be released with each hit.

The player starts with 20 shots, and the game ends when they reach 0. The player receives an extra shot if they destroy all of the targets on a level with only one shot and two extra shots when they pass a specific number of points. This amount starts at 250 and doubles every time it is passed.