Naruto and Sasuke were huddled together in a bush, not too far away from the ending fight, when they looked upon someone else. Further from the two fighting ninja, a person hid in waiting.
Sasuke had been the one to notice as they had slowly backed up and moved Tazuna away from the fight. They connected that the man was lying in wait for Zabuza to either win or come close to losing. The ninja wore an Anbu-like mask that was not shaped like an animal. The man wore a pinstripe haori, and he had senbon at the ready. As they watched intently, they barely saw the ninja move. However, that movement was enough for Naruto.
He threw a set of shuriken at the flying senbon. Meeting them in mid-air, the metal weapons clattered against each other and hit the ground. That's when Naruto and Sasuke chose to reappear.
Kakashi barely noticed the presence until he was about to finish Zabuza. Seeing the senbon get stopped by shuriken, he saw that as his cue. He dodged under a hastily thrown cross punch and sliced open the man's stomach. Zabuza began to back up, looking to the other ninja to support him.
The ninja tried. However, Naruto distracted the person temporarily, throwing a barrage of kunai in Kakashi's direction. This forced the other shinobi to keep in his place, and this allowed Kakashi to leave. Kakashi then reappeared behind Zabuza and slit the missing ninja's throat.
However, as he looked down, he saw the masked ninja. Blood spurted from the boy's throat, but he was given no rest. A second later, the massive blade of Zabuza split the ninja's body in half. Kakashi moved away immediately, seeing the blade coming for him. Flipping Kakashi, landed and took on the enemy ninja. The man was brutally battered. He was bleeding from his abdominal injury, and his eyes seemed to be tearing up. Zabuza's chin wrapping was slowly falling, and so Kakashi decided he had just enough chakra for one last jutsu.
[Lightning Style: Raikiri]
The sound of a thousand birds chirping rang through the mist-laden forest and lake. Naruto and Sasuke, who had just arrived, looked at their sensei, seeing the jutsu he had. Kakashi seemed to disappear and reappear with his hand through Zabuza's chest before the man could even react.
Naruto and Sasuke's eyes widened in horror as they watched both the enemy ninja and their sensei go limp and fall onto the ground.
After a second, Naruto was the first to snap out of it, grabbing his sensei and asking Sasuke to grab the massive blade. The young Uchiha could barely lift it, but they ran off in the direction of Tazuna's house. Naruto made another clone to grab Zabuza's body just in case he had not died.
The two boys arrived at the home of Tazuna and brought Kakashi to rest. They also brought in Zabuza after some considerable convincing. They met Tsunami, the daughter of Tazuna and Inari, the man's grandson. Naruto noted that Inari was a depressing little child, but his mind was not focused on it.