One of those rare well-written "Naruto is revealed to be a girl under a long-term transformation" stories.

In this case, the plot device is a cleverly creative one that, when Naruto taps into the Kyuubi on the bridge in Wave Country, Haku's control of the mirrors slip and they revert to their base behaviour of "reflecting the truth"... which grants both Naruto and Haku a harrowingly honest view of who and what they and each other are... in the process, stressing Naruto enough to break the transformation.

This was a difficult story to rate. It's well-written and the gender-bending is central to the story, but there's so much development of the surrounding characters and setting that, within what's been written, the best way I could represent the gender-bending was to say that it's shared equally between two minor incident classifications.

...which means that most of the text feels just fine if you gender-swap Naruto and Haku's pronouns back to the more typical "male Naruto gets female Haku" combination. This also means that time progresses very slowly with around 40,000 words of character interaction between Naruto changing on the bridge and arriving back in Konoha.