The Assassin of the Mist!

is episode 7 of the original Naruto anime.

Summary
Team 7 is on a motorboat, owned by Kaji, headed towards the Land of Waves. Kakashi threatened to abandon the mission if Tazuna did not provide an adequate explanation for lying about the mission's difficulty level. He told them that an extremely wealthy businessman called Gatō was responsible for his country's struggles, and could not afford any high ranked missions. Later as they were walking, Kakashi suddenly told everyone to duck just before a giant sword spun over them and a ninja appeared on the handle of the sword. The ninja revealed himself to be Zabuza Momochi, a missing-nin from Kirigakure. Zabuza demanded to have Tazuna, to which Kakashi refused. When Kakashi recognised Zabuza, he was forced to use his Sharingan. Sasuke immediately recognised it and explained, along with Zabuza, that the Sharingan is a dōjutsu that can immediately see and anticipate all ninjutsu, genjutsu, and taijutsu at hand. Sasuke was confused, as only members of his clan wielded the Sharingan. Zabuza summoned a thick veil of mist, and Kakashi informed his team to protect Tazuna. The great tension between the two jōnin causes Sasuke to have a panic attack, but Kakashi is able to calm him down. Zabuza stated that it was over, and prepared to strike them all with his sword. Kakashi raced toward them and stabbed Zabuza with his kunai just in time, but Zabuza revealed that he merely used a water clone. He then sliced Kakashi in half, but Kakashi, who also used a clone, appeared behind Zabuza, catching him off guard, and held a kunai to Zabuza's throat.

Trivia
In the manga Tazuna says his grandson is 10 years old, in the anime he says he's 8.

Убийца из тумана!