Today is a big day for you: the Digimon finally managed to open a stable portal so you can travel freely between File Island and the real world. But this is not why you're so excited: your best friend Taku is coming to visit your house on File Island! Among the things you want to show him is the collection of Digi-items your Digimon team gathered in the numerous battles they faced.
After getting acquainted to your Digimon friends, you take Taku to the room full of shiny and ancient artifacts, and it's a success: your friend is amazed by your strength and asks you about each item on display. But you can clearly see that his gaze rests on the B Spirit of Fire you got after defeating Vritramon, so you tell him everything about it and its powers.
"I also wish I could have such powers..." says Taku before you all go sleeping.
It's the middle of the night and your sleep is interrupted by loud bangs. Opening your eyes, you realize that it's not a knock on the door, but that it's rather coming from outside.
Rushing to the window, you see a glowing figure in mid-air, hurling fireballs at the house:
"Come out and face me! I will take your place as the ultimate tamer of File Island. You can't even Digivolve like I did! Thank you for the invitation, byt the way, ha ha ha!"
What the Digimon just shouted made absolutely no sense to you. Humans don't Digivolve, so that is certainly not Taku, as it had you believe.
"Tamer, make no mistake! that IS Taku right there!" a teammate roared after rushing outside. Another one steps in and says to you: "Somehow he managed to Digivolve to Agnimon by using the H Spirit of the Flame. He might have conjured it through his intense wish of having your powers!"
You are reluctant to start battling your dearest friend, but he is unable to grasp the responsibility a true Tamer has, and certainly not the tremendous powers granted by the Digivolution.
"OK team, we have no choice other than defeating Agnimon in order to convince it to return back to his human form. I want my friend back, let's go!"