"The Weapons Known as Shinobi" (忍という名の道具, Shinobi to Iu na no Dōgu) is episode 18 of the original Naruto anime.
Inari runs door to door throughout the Land of Waves trying to rally the villagers to defend Tazuna and the bridge. To his disappointment, the adults are too afraid to get involved, but Inari is willing to go to the bridge alone if necessary. At the bridge, Kakashi has immobilised Zabuza with his Ninken and readies his Lightning Cutter for a finishing blow. Meanwhile, Haku, having been defeated by Naruto, has seemingly lost his purpose in life (serving Zabuza) and requests that Naruto kill him. Naruto is confused and argues that being a shinobi is more than just mindless killing, but Haku won't be dissuaded. Naruto relents, saying softly that they might have been friends under different circumstances, and charges at Haku with his kunai drawn. However, Haku senses that Zabuza's life is in danger and blocks Naruto's attack. Declaring he's not ready to die yet, he rushes to Zabuza's side just in time to take all the damage of Kakashi's Lightning Cutter. Back at the village, Tsunami tells Inari that he is too young to fight, but he is determined to defend his village. His demeanour reminds Tsunami of Kaiza, which surprises her. Inari leaves to go protect the bridge but finds a crowd waiting for him outside. As Zabuza's mist clears, Naruto is stunned to find Haku killed by Kakashi's attack. Zabuza is unfazed by his partner's sacrifice and swings his sword to slice through Kakashi and Haku at the same time. Kakashi evades this blow and prepares to resume the fight, telling an enraged Naruto to stay out of it. Tazuna accompanies Sakura to Sasuke's body. The girl tries to maintain her composure as per shinobi requirements but breaks down in tears when she sees that Sasuke is cold and lifeless. Kakashi once again immobilises Zabuza, this time disabling both of his arms. With Zabuza unable to swing his sword or perform techniques, the battle seems over. Gatō appears with dozens of hired thugs and tells Zabuza that he's fired.
In the anime, Kakashi's blow only strikes Haku on the chest. In the manga, his hand goes straight through Haku's body.