Digimon Adventure 02: Tag Tamers was based on the manga series Digimon, in which children can travel to a digital world and help good digital monsters to defeat evil ones.
The game starts off with Ryo visiting Ken Ichijouji in his room. They witness Diaboromon's defeat by Omnimon on the Internet, and Veemon appears from the computer with a D-3 for Ryo. There was an extra Diaboromon loose in the Digital World, and Ryo is called in to defeat it. Ken follows Ryo , only to be left behind in Primary Village with Taichi and Agumon to wait for Ryo's return.
Tag Tamers was developed and published by Bandai (now Namco Bandai Games) as the last Digimon game for the WonderSwan console.
Source: Wikipedia, "Digimon Adventure 02: Tag Tamers", available under the CC-BY-SA License.