A Plea from a Friend

is episode 133 of the original Naruto anime.

Synopsis
Sasuke is dumbstruck by Naruto's display of power and asks Naruto what he is. In tears, Naruto says that he is Sasuke's friend and will not let him defect to Orochimaru even if it means breaking Sasuke's arms and legs to stop him. The boys fight, and Sasuke struggles to counter Naruto's increased speed and strength. After a severe beating, Naruto pins Sasuke against the rock wall and asks him if he has come to senses yet. Sasuke tells Naruto that he could not possibly understand what it feels like to lose his whole family, because Naruto was alone from the start. Naruto admits that whereas he knows nothing about parents or siblings, he thinks of Iruka as a father, and he wonders if his relationship with Sasuke is similar to having a brother. Naruto states that to protect this bond, he will stop Sasuke no matter what. Sasuke resists Naruto's appeals and and says that it is too late for him to go back. He puts on his forehead protector and calls for Naruto to come, that he will break the bonds of their friendship. A third tomoe appears in Sasuke's Sharingan signifying that it is now fully mature. Naruto asks Sasuke why he's bothering with the headband if he seeks to break his bond with Naruto and the rest Konohagakure. Sasuke explains that he is recognising Naruto as an equal, though he once again taunts that Naruto won't even be able to scratch his forehead. Elsewhere, Kakashi catches up to Pakkun and the two hurry to stop Sasuke and Naruto. Naruto and Sasuke resume their battle, but this time Sasuke has the upper hand, harnessing the full power of the Sharingan. Naruto uses a chain of shadow clones to throw Sasuke into the cliff wall, but Sasuke counters with Fire Release: Dragon Fire Technique. A half-conscious Naruto again implores Sasuke to stop, but Sasuke declares it too late and as a finishing move, pile drives Naruto into the ground below. Inside Naruto's subconscious, the Nine-Tailed Fox says that Naruto is too weak and that the boy should be thankful to him. Seemingly defeated, the fox's red chakra begins pouring out of Naruto, who slowly gets up and punches the shocked Sasuke. Naruto grows a chakra tail, moves like a fox, and attacks Sasuke once more. Sasuke struggles against Naruto's onslaught and notices that the red chakra seems to have a mind of its own. Sasuke tries using his cursed seal, but even that is ineffective against Naruto's attacks. Sasuke laughs and says that Naruto has given him no choice but to use the second level of his cursed seal. As the seal transforms his body, Sasuke's hair grows longer, his nails extend, and his skin turns purple. He acknowledges that Naruto possesses a special power, but says that he is more special.