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Okay, first things first—this is not only a Crossover, it is a Crack fic. It is MADE of crack.
Essentially, baby-Naruto gets thrown between worlds and found by the Joker and Harley (DCAU versions). They decide to raise him (and Harley insists they learn some Japanese because the baby needs to learn his heritage). Later they are all pulled into Konoha and the Joker realizes he can do his normal stuff (torture, poison, kill) and it's A-OK if he does it in the name of the village. He decides to join up with the Interrogation Unit.
Also Harley makes hammer-space a jutsu. The Joker's blood being toxic and resistant to poison? Yes, that becomes classified as a bloodline. And Naruto—well, he's insane. Happily so. And he wants to put a smile on the world's face. There is Fish Fu. I'm not even kidding.
Now, as for warnings, there is some Squick, and the author does take gleeful advantage of the Values Dissonance between Konoha and the DCAU (and the real world) in writing the story. Naruto is a Heroic Sociopath and he seems interested in getting his teammates that way too. Fans of the heroes of Justice League might not want to read the chapter they're in—it has little to no bearing on the plot and is just Joker manipulating things in Konoha to screw with them.
I think one of the best things about the fic is how it handles language. Everyone is always speaking Japanese unless it's between inverted parenthesis )like this( in which case it's English. Also, the Joker emphasizes random worlds, giving his madness a more tangible feel. The dialogue feels really good (especially for a Crack fic where a lot of times dialogue is only there to move the plot instead of be dialogue), though Lampshades abound at every turn.
All in all, amazing for what it is. It'll...put a smile on your face.
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