A fourteen-year-old youth named Marcus Damon, who desires to be the best fighter in the world, meets a strange dinosaur-like creature and engages the creature in a fist fight that ends in a draw. At that time, after introducing himself, Agumon vows to be Marcus's follower as the two are confronted by Yoshino Fujieda, a teenaged member of a government agency known as DATS dispatched to capture Agumon. After escaping and hiding Agumon in Tokyo Tower, Marcus is found by Yoshino who brings him to her superior Commander Sampson. Commander Sampson and his Digimon Kudamon explain to Marcus that Agumon is a wild creature that will go out of control. Though Marcus dismisses this with the intent to bear any responsibility for anything Agumon does wrong, an alarm of a burger joint being attacked is reported in the area Agumon was hidden in. Though it appeared Agumon was the cause, it turned out to be the work of a Kokatorimon. After Kokatorimon knocks Agumon out, a furious Marcus sprints towards Kokatorimon and punches the bird in the face. Astonished of the energy generated from his fist, Marcus encounters a mysterious old fisherman who gives him a Digivice to channel his charged DNA in so Agumon can become stronger. This causes Agumon to Digivolve into GeoGreymon, defeating Kokatorimon as it reverts him into a DigiEgg. Commander Sampson is impressed by the turn of events.

Brought to DATS HQ by Yoshino, Marcus and Agumon learn that wild Digimon are placed in a teleporter to be sent back to the Digital World. With DATS still seeing Agumon as a threat, Marcus stops them from forcing Agumon into the teleporter and the two escape. Marcus resolves to hide Agumon at his house, only for the Digimon's existence to be found out by his mother Serah Damon and his younger sister Kristy Damon. From there, to Marcus's annoyance, his family and Agumon all quickly become friends. Marcus is further annoyed when he learns that Yoshino is going to keep an eye on him and Agumon indefinitely unless the youth becomes a member of DATS. The next day, Marcus passes by Kristy's school and learns that something is attacking the school's animals. Marcus plans a stakeout of the cages and places Agumon inside as bait, the culprit revealed to be a Kunemon that has been feeding off the emotions of Kristy's young classmate Takashi who secretly wished the animals would just disappear. No longer needing Takashi, Kunemon attacks with Yoshino's Digimon partner Lalamon digivolving to Sunflowmon to fight the wild Digimon. However, Kunemon digivolves to Flymon and uses his poison powder to paralyze Sunflowmon and Yoshino. By that time, have been unable to digivolve Agumon, Marcus learns how to access his DNA and activates it by punching Flymon before it is defeated by GeoGreymon. The old man who gave Marcus his Digivice then appears, telling the youth of the Digital World and all the opponents he could fight there. Returning to DATS HQ, Marcus is welcomed by Commander Sampson as a new member of DATS.

At the Damon household, everyone is preparing themselves for the day ahead when Marcus receives a call from DATS that a DemiMeramon has begun attacking some men who. However, due to DemiMeramon's lack of body mass, neither Agumon nor Marcus could do any damage which makes the latter unable to activate his DNA. As the two run off after DemiMeramon after he flew off, a blond-haired youth arrives and wipes men's memories of the attack before going after the wild Digimon. A while later, unable to find DemiMeramon, Marcus learns the wild Digimon was captured by the blond-haired youth who is identified as the child genius Thomas H. Norstein who has returned after six months of being at DATS's Austria branch. Because of their different personalities, Marcus and Thomas get on each other's bad side as they end up having a boxing match before learning that DemiMeramon spawned from original's flame sparks are attacking the city. Sent by Commander Sampson to battle the DemiMeramon, Thomas has his partner Gaomon digivolve into Gaogamon to defeat them. However, three DemiMeramon remain at a chemical plant close to DATS. Starting to suffer self doubt after seeing Thomas and Gaomon fight, Marcus runs off before finds the old fisherman cooking a fish on a makeshift stove, the man's speech of fanning flames giving Marcus an idea. Confronting the DemiMeranon, Marcus has Agumon use his fire to strengthen them until they combined and digivolve into Meramon. Marcus, now having an opposite he can hit, is then able to punch Meramon and has GeoGreymon finish the wild Digimon off. Later, to both their dismay, Marcus learns that he and Thomas are to be teamed up.