Her parents are being obnoxious. It's 'dangerous outside' and 'no place for a child' and she should defend the evacuation tunnels blah blah blah.
That's all boring nonsense, so Hisame decides that she isn't a Uchiha anymore so it being un-Uchiha to ignore her parents doesn't count and she doesn't have to do what they say, which leaves her free to use her awesome ninja skills to sneak past the terrified and harried genin guarding the shaking caverns.
The world outside is, indeed, rather dangerous, and more than a bit chaotic. But there's a floating ring of light where the moon used to be, and the trees are fighting strange white figures, and when she climbs to the very top of the Hokage Monument she gets treated to someone throwing a METEOR at someone else and being stopped by a sudden MASSIVE TREE.
Suspicion confirmed: this is so much cooler than hiding in a tunnel.
Hisame secures her kunai better - she might get to stab an enemy, that'd be cool - and goes to hunt for a better view.
(She's rather put out to discover that the enemies are not very stabbable, but at least most of the people on her side are ignoring her except for some harried shouting telling her to hide. Usually before the enemy stabs them 'cause they got distracted yelling at Hisame, which, what did they expect to happen?)
Tragically, wandering around a battlefield the size of a continent when you're an Academy student very quickly gets also boring, which leads to Hisame stomping around and contemplating if she's technically a defector since she stopped being an Uchiha, and if so if she's therefore the enemy of all the shinobi running around, which means she can probably stab them.
If her parents were around to watch her, they'd be getting very, very nervous right about now.
- And that's when a strange glowing figure begins floating in front of her, and he says something about being the father of her first incarnation, and the shinobi will lose but he can give her a gift to grant an edge, his eyes and chakra -
Controlling gravity is SO MUCH FUN. Hisame isn't sure what else the super cool eyes do but she can definitely send people flying and make craters. She hasn't gotten the trick of controlled flying yet but she's working on it. And zooming chaotically through the air is fun, too.
- It turns out that 'what else the super cool eyes do' is 'let her survive when the entire world crumbles.'
Hisame looks around at the empty void the world is dissolving into. Looks at herself. Looks at the weird woman who isn't dissolving, the one with the old fashioned robes and horns who looks kinda like the glowing floaty guy. She looks at her knife. Back to the woman, who's glowing lots and lots more and starting to grow, and also totally ignoring the small child standing behind her.
Can she stab this enemy? Well, she's gonna find out!
The answer is: yes.
Also, apparently weird glowy people explode when you stab them, instead of bleeding, and now Hisame's ears are ringing and she's laying flat on some hard earth with her knife held in a death grip in front of her.
"...Ow," she says, belatedly.