In this title of the Custom Robo series, each player is called a "commander", who pilots and customizes a miniature "robo" to battle each other in a virtual arena called a Holosseum; the average robo is 32 centimeters tall. The first half of the story takes place in "A New Journey" mode. After the story mode is completed, the player may choose to go through the story again or enter a series of tournaments called "The Grand Battle".
The main objective of the entire game is to engage in battle with other robos, and finish its plot by collecting every custom robo, battle part, and item that the player wants, while winning every battle that moves you from one part of the storyline to the next. In Custom Robo battles, the objective is to reduce your opponent's hit points from 1000 to 0 by using different custom robos and their dash attacks, guns, bombs, and pods.
Source: Wikipedia, "Custom Robo (Nintendo GameCube)," available under the CC-BY-SA License.