Okay soo- there's two ways to interpret this. An anime about an anime(er, the animated world.) Or A manga about an anime. Or the real world about an anime. I guess to sum it up, think about it like when the fake world come in contact with reality.
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With a rip in the animation world, Len ends up in reality with two crazed otaku's, Yuri and Yuuki. Yuri of course knows that he just ended up there by accident through Yuuki's insane cellphone thats filled to the brim with anything and everything anime or manga, including vocaloids. But, Yuuki sure doesn't know that. Taking Yuri's advice to keep the fact he's not a normal human a secret, how's Len going to put up with hiding it from Yuuki when he falls in love with her? And even worse- he might have to go back into animation and leave everything behind. Len ends up being caught in Reality.