An Old Nemesis Returns

is episode 265 of the Naruto: Shippūden anime.

Summary
After the battle against the Akatsuki's Commando Unit, the Allied Shinobi's Commando Unit continue to set paper bombs against the enemies. Sai, Zaji and Omoi then prepare the signal for the Third Division lead by Kakashi. Haku, Zabuza, Gari, Pakura and Toroi gets hit by the paper bombs to which Haku counteracts with an ice barrier. Zabuza then states to Haku that he just wasted the technique due to the Impure World Reincarnation's ability to regenerate the resurrected whenever damaged. Haku reads the movements of Sai's ink birds and strikes them using his Demonic Ice Mirrors forcing them to a one-on-one combat. As the group strikes, Kakashi, Guy, Lee and a shinobi intervene the battle which shocks both Haku and Zabuza. Zabuza then states that this is neither heaven nor hell to which Kakashi responds that they are in the world of the living and that they are being manipulated by the Impure World Reicarnation. As the Third Division wait for their captain's signal, Sakura remains in shock to see Haku and Zabuza. Zabuza then states that Sakura has grown up and questions her if Naruto grew up to be a fine shinobi as well. Haku then reminisces the time he met Naruto in the woods. Kakashi responds stating he grew up to be an admirable shinobi because of the both of them. Haku then states that he will become even stronger. Elsewhere, Kabuto is seen manipulating the technique. He then uses his special tag to erase their memories to turn them what he calls "killer puppets." A huge amount of aura is released as Haku, Zabuza, Gari, Pakura and Toroi get their memories removed. Amidst the transformation, Haku and Zabuza continue to cling to their personality much to Kabuto's dismay. Haku states to Kakashi that he is sorry that he has failed to protect his master and that he failed to fulfill his dream which Kakashi strongly responded that he didn't fail at all. Zabuza then remembers the time he was lectured by Naruto on how Haku really felt about him. Haku then states his last words to Kakashi saying to "stop them once again" before they fully succumb to the transformation. As the battle begins, Zabuza activates his Hidden Mist Jutsu to use his Silent Killing technique as the shinobi prepare for the attack. Zabuza then silently kills some shinobi, Pakura uses her technique which could mummify her enemies as Gari blows up Tajiki. One shinobi angrily charges at Gari as Haku appears out of his mirrors and is stopped by Guy and Lee's combo before he could even react. Guy then tells the newbie to cool down as anger would only bring more problems. As the battle progresses, Kabuto summons six more coffins to the battlefield as Zabuza retreats. As the coffins open, Kakashi and his division were shocked to see the six remaining members of the Seven Ninja Swordsmen enter the battle.

Trivia

 * In the Naruto original series, Zabuza requested Kakashi to place him by Haku's side. In this episode, Zabuza approached Haku instead while dying.