The series takes off while a group of Kids are at Summer Camp, they incidentally get pulled into a place known as the Digital World, that was populated by creatures called Digimon. They were given devices called Digivice and paired with a monster, and fought to save the digital world.
In the first season it starts off when a group of friends are pulled into the digital world. Each of them a paired with their own partner.
Throughout the season they are in search of crests. The crests give the Digimon the power to digivolve to higher levels of power. In the series Agumon is the first of the Digimon to digivolve into a champion level Digimon. After he digivolved became Greymon. In the series it is revealed that there is an eighth Digidestined. When this is discovered they need to travel back to the real world. During this time Myotismon also goes in search of the digidestined. Later it is revealed that Kari, Tai's little sister is the eighth digdestined. When they have to defeat Myotismon none of the Digimon are able to stop him, but then Agumon and Gabumon warp digivolve into WarGreymon and MetalGarurumon. After they defeat Myotismon they have to return to the digital world to defeat the Dark Masters, a new group of evil Digimon trying to take over the world. Once the Dark Masters were defeated they had to go and face their greatest enemy Apocalymon, the Digimon made of all of the evil Digimon they had already faced. They were only able to defeat Apocalymon when they combined the power of all their crests.
The loss of the crests in the battle with Apocalymon prevented the digimon from being able to evolve past champion. It wasn't until Azulongmon, a celestial God Digimon, intervened in the 2nd season of the show that the original digimon, along with the new wave of digidestined discovered, where able to transform into ultimate and, in the case of Agumon, into mega levels.
Subsequent seasons do not rely on the use of digi-crests to spur digivolution. In season 3, cards where substituted and in seasons 4 and 5, gears resembling digi-vices where used to link up humans with digimon to create powerful hybrids. Post season 3, none of the digimon subsequent seasons or series are tied into each other, thus the original team has not been seen since outside of tie-in films and media.