The Weapons Known as Shinobi

is episode 18 of the original Naruto anime.

Synopsis
Inari is running from door to door all throughout the village trying to get everyone riled up to defend both Tazuna and the town. But to his disappointment, no one wants to get involved. Even Giichi questions just what it's worth to get involved. Inari won't be deterred however, and declaring that nothing will be solved by running and hiding, runs off alone to get ready.

Back at the bridge Kakashi is still squared off with Zabuza, who is still under the grip of Kakashi's nine dogs, which he summoned. Kakashi lists each of Zabuza's crimes leading up to the present and proclaims that he no longer has a future. He begins to build up his chakra to do his Lightning Cutter technique and all Zabuza can do is weakly struggle against the dogs.

Meanwhile, Naruto and Haku are still squared off with Haku telling Naruto over and over to kill him and become a great ninja. Naruto gets angry, shouting back that being a shinobi is more than just mindless killing. Haku remarks that they are a lot alike and once again asks Naruto to kill him. This time Naruto relents, saying softly that they might become friends under different circumstances. He draws his kunai and after swearing vengeance for Sasuke rushes at Haku. Haku stands there, ready to accept the finishing blow.

However, Kakashi has built up enough chakra for his technique and rushes at a defenceless Zabuza. Haku quickly becomes aware of this and halts Naruto's attack. Declaring he's not ready to die yet he does hand signs to set up his Demon Ice Mirror before teleporting. Everything literally happens in slow motion after that. Neither Kakashi nor Zabuza see the Demon Ice Mirror or Haku's sudden appearance. Haku makes it in time to absorb all of Kakashi's Lightning Cutter and Zabuza makes it out of the attack in one piece, though the two parties are only aware of what's going on after the blow is landed. As the lightning fades Haku coughs up a lot of blood and using the last of his strength grabs hold of Kakashi's arm, hissing Zabuza's name softly.

At the village, Inari is getting ready to charge the bridge with Tsunami chastising him the whole way. But he still won't be deterred and swears to no longer hide and to defend his village. His demeanor reminds Tsunami strongly of Kaiza and it takes her off guard long enough for Inari to make it out the front door. But there's a crowd waiting for him outside.

Naruto is completely confused by what's going on, but he's able to see what's going on now that the fog's lifted. He sees both Kakashi and Zabuza standing in front of a dead Haku who's still dripping blood onto the ground. Kakashi is completely stunned by Haku's sacrifice, but Zabuza laughs. He swings his sword at Kakashi, intent to slice right through Haku to kill Kakashi and Haku's literal death grip on Kakashi's arm makes it difficult to move. But Kakashi makes the decision to grab Haku's body and jump away from Zabuza before both are sliced. Kakashi closes Haku's eyelids and arises to continue the fight, telling an enraged Naruto to stay out of his fight.

Sakura, who had been guarding Tazuna, sees Naruto and runs over to him asking about Sasuke. Naruto chokes up, remembering Sasuke's sacrifice during their fight with Haku, but he can't bring himself to tell Sakura what happened. Tazuna offers to take her to Sasuke, since she would not be violating her job to protect him then. They reach Sasuke's body and Sakura tries to keep her composure as per shinobi requirements. But as she laments her feelings for Sasuke she loses control and clings to his body, sobbing hysterically.

Kakashi and Zabuza continue to fight, Kakashi seeming to be furious while Zabuza doesn't seem all too bothered about Haku's death. But Kakashi nevertheless gains the upper hand in the battle and managed to disable both of Zabuza's arms, immobilizing them so he can no longer swing his sword or perform jutsus. The battle seems over then.

But then, Gatō shows up on the bridge with dozens of hired thugs in tow. He nonchalantly tells Zabuza that he's fired.

Trivia

 * In the anime Kakashi's blow only strikes Haku on the chest. In the manga his hand goes straight through Haku's body.