It's canon to the Naruto Shippuden.

Sakura,Naruto and Sasuke are teenagers.

They were training with Kakashi like usual.

But,while training,something happended..

While they all were in a forest,training,someone used a jutsu on Kakashi and fled.

Kakashi got turned into his 15 years old self and he was dumbfounded by who the hell people are,where the hell was he,and how the hell did he get there.

Sakura,Naruto and Sasuke felt responsible for their sensei's situation and tried their best to take care of him.Which,actually,turned out easy for Naruto at least.

When Kakashi saw how much Naruto looked like his sensei(Minato),he got imediately attached to him.

The team decided that they needed to keep him entertained,so they started thinking of ways to do so.

After a moment,they took him to Tsunade to see him.

(You're Naruto.)