You meet Kenji, who teaches you about ganbaride (you didn't know what ganbaride was before this). He tells you about an arcade elsewhere that also has a ganbaride machine and suggests you investigate it. You go there and see a guy dressed in black (later known as a Bug) stealing a rare card (apparently he's invisible but for some unexplained reason you can see him).
You follow him to that underground place (you know the one) and he gives it to Karen. You tell her STEALING IS WRONG and she uses her Gear (looks like a deca-driver) to summon Kiva and Ixa. A mysterious guy/girl (I CAN'T TELL!!) appears out of nowhere and gives you a Gear saying he/she wants to see which is stronger, justice or evil. Judging by the character portrait, you apparently put it on your wrist (ur doin it wrong!!). You fight Kiva+Ixa but Karen got away. Next week, you're at DOLL (paranormal investigation's hq). A girl named Sara asks you to investigate another girl at her school named Karen (you didn't know her name when you fought her) who has been getting 100s on her tests recently. This is weird because Karen is a retard that when asked what's 3 times 3 answers 18. She hasn't been cheating because Sara sits behind her and can see if she does anything.
Professor Hiroto theorizes that Karen has a Doppelganger that's smarter than the original Karen and is helping her by taking her tests for her. You go to her school but can't find Karen so apparently you have to wait 3 weeks for another test to be taken (you can advance time by choosing the 4th option at DOLL). After 3 weeks Hiroto starts to doubt his theory, as a doppelganger would try to replace the original.
You find Karen (who you now recognize as the girl you fought before, though you remain silent about it) and Hiroto uses his paranormal scanner to find out that she's human. Hiroto asks her how she's been getting perfect marks on her tests. She says there's nothing weird about it and can even recite the all the answers (was I the only that thought it was weird that she memorized them). Hiroto asks her what's 3 times 3, she answers 18. Unable to find out how she does it, you have no choice but to let her go.
Next week, you meet Kenji again. He's wondering how good your ganbaride skills have gotten. After your rematch with Kenji, he gets a new card. It's Den-o. You wonder who that is because you are a newfag. Kenji explains Den-o's premise and you figure out the Karen is using time travel to find out the test answers in advance. Apparently you have no confidence to beat Karen, so you train for 2 weeks.
You go to Karen's school and see Sara and Karen arguing. Karen tells Sara it's none of her business what she gets on tests and to leave her alone. After Sara leaves, you confront Karen and tell her you know what she did. She says she's done nothing bad, her parents are happy she's getting good grades. You tell her it's not right to get those grades unfairly. She doesn't care about that, she says she also uses the den-liner to go to the future to buy games and manga that aren't released in the present (I wish I could do that...)
You and Karen agree to settle things with a ganbaride battle, loser loses their Gear and leaves the winner alone. You go to the park where nobody will see you battle. After you beat her, Doctor Hades appears out of nowhere and takes Karen's gear. He says he didn't give her the Gear for her petty personal problems, he gave it to her so she could perform EVIL. He disappears in a flash.
There was some other stuff mentioned like a person named Seraph that I think is the person that gave you your gear but I can't remember the specifics even though I just played this part 3 hours ago, sorry.