Naruto and Sasuke are both injured severely from Haku’s Crystal Ice Mirrors attack. Haku launches an assault on Naruto, but Sasuke dives in to sacrifice his life for him. Naruto tries to avenge his death, by transforming into the Nine-Tailed Fox Demon and charges up to kill Haku, while Kakashi worries about his supernatural powers.
Source: Wikipedia
Naruto Episode 16 – The Broken Seal