Who Are You?

is episode 343 of the Naruto: Shippūden anime.

Summary
Naruto enters Tailed Beast Mode and attacks Tobi. Meanwhile, a hesitant Kakashi is in disbelief of the masked man's identity but Guy ultimately snaps him out of it. As the battle rages on, Kakashi is finally motivated to fight and prepares to join the fray. Tobi unleashes a shower of gigantic chakra receivers, paralysing B and Gyūki. Naruto creates a shadow clone and it charges forward with Rasengan while the original prepares a Tailed Beast Ball with Gyūki providing cover. As Naruto launches his technique, Tobi simply phases through and with that, fully enters the other dimension. There, he notices the clone from earlier and realises that Kakashi did not just teleport the Rasengan but the clone itself. With little time to react, Tobi is hit by Naruto's Rasengan; his mask shattered into pieces. Watching on, Kakashi and Guy are shocked to see that the masked man is their former comrade, Obito Uchiha, and flashback on Obito's history. Rendered speechless and completely at the news that his supposedly dead friend and former team-mate is alive, Kakashi is shocked that Obito survived. Naruto questions who the individual is, to which Guy responds that he is their former comrade who was supposedly killed in action during the previous war and who was also a member of the Uchiha clan. In response to Kakashi's question, Obito explains that the reason why he hadn't revealed himself up until this point was because he let their former team-mate, Rin Nohara die. Naruto motivates a devastated Kakashi as Obito's plan to conquer the world needs to be stopped with Guy joining in as well and telling Kakashi that they need to fight. Obito attacks the group with his Fire Release: Blast Wave Wild Dance technique but Naruto intercepts with Kurama's tails. Just then, Madara appears alongside Obito, much to the others' surprise.

Trivia

 * When Obito is panting during his training for his second Chunin Exam, his nose disappears.