In what episode does guy open the 8th Gate?
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The episode continues from the previous with a flash-back of Guy getting accepted as an alternate student. After acceptance he realizes his weakness, and decides to train himself vigorously in order to challenge his new rival Kakashi.
With encouragement from his father, Guy makes good progress and graduates from the Ninja Academy. When Guy starts going on missions, he could not help noticing his father doing menial tasks and he became embarrassed and even hid from him at one point. Later on he is reassured by Kakashi that his father is a cool guy that raised him to be what he is.
One night his father brought him to the woods to pass on a technique to him he has been working on for 20 years. It was the forbidden 8 Gates Formation. Later on when Guy and his team faced the Seven Ninja Swordsmen of the Mist, Duy arrives just in time and sacrificed himself by activating the 8th gate to save them. The flash-back ends with Guy in tears (because of the memory). The episode ends as Guy charges toward Madara and activates the 8th inner Gate.