"White Past: Hidden Ambition" (白い過去・秘めた想い, Shiroi Kako - Himeta Omoi) is episode 17 of the original Naruto anime.
Naruto's chakra rages around him wildly. Haku remarks that chakra usually couldn't be seen, yet it was clearly visible. Naruto, still angered by the "death" of Sasuke, had weakened the seal that kept the Nine-Tailed Demon Fox's chakra in place. Surging with power, Naruto healed his wounds and attacked, brutally overwhelming Haku, and deflecting his needle attacks with waves of pure chakra. Haku attempts to flee the circle, but Naruto sees him and snatches his arm with the speed of the Nine-Tails.
Meanwhile, the battle between Zabuza and Kakashi continued. Kakashi was relieved to sense that the seal for the Nine-Tails had only weakened, and had only let a small portion of the fox's chakra out. He decided to end the battle quickly. Zabuza was curious as to how Kakashi would do that when he couldn't find Zabuza. Kakashi's only response to this was to pull out a scroll.
Naruto, having grabbed Haku's arm, delivered a powerful punch, sending Haku through one of the mirrors and, through that process, destroying them all. Naruto rushed at Haku again, ready to deliver another enormous punch, but, at that moment, Haku's mask came off, to reveal the boy he had met in the forest. This caused Naruto to pause in his attack. Haku asked Naruto to kill him, because he was no longer useful to Zabuza, calling himself nothing more than a broken tool. Haku then went on to explain his past; about how he was born in a small village, and how his parents dearly loved him. However, during that time, anyone with a kekkei genkai, a bloodline trait, was feared. When Haku was a small boy, he discovered that he had inherited one from his mother. Unfortunately, his father discovered this, and killed Haku's mother. Just as Haku's father was about to kill him too, Haku tapped into his unknown powers and killed his father. A little while after that day, Zabuza found Haku, and took him with him to train him in the most advanced ninja skills.
Kakashi finally used the scroll he had pulled out earlier to use the Summoning: Earth Release: Tracking Fang Technique on Zabuza in order to track him. Kakashi revealed that, even though he could not see or hear Zabuza, he would be able to follow his scent. He also revealed that, earlier, he had let Zabuza cut him so that Zabuza would be covered in his blood, and therefore easier to track. Kakashi then said he would introduce Zabuza to his one and only original technique: the Lightning Cutter.