"The Mask that Hides the Heart" (心を隠す面, Kokoro o Kakusu Men) is episode 349 of the Naruto: Shippūden anime.
The Ino–Shika–Chō trio are sent on a mission to deliver a treaty to Iwagakure. However, the exchange almost turns into an altercation until the Fourth Kazekage arrives to intervene, much to the Iwa-nin's surprise which causes them to flee. The treaty is brought before the Third Tsuchikage and his council who discuss the future. Meanwhile, Kakashi is still plagued by nightmares of Rin's death, leading the young man to become even more sullen and withdrawn. Meeting with the Konoha Council and Danzō Shimura, Hiruzen discusses the village's future and his decision to step down as Hokage and allow the next Hokage to usher in a new era for the village. At the meeting with the Fire Daimyō, Hiruzen rejects Danzō's nomination of Orochimaru to the position in favour of Minato Namikaze who had distinguished himself in the last war. As Danzō and Orochimaru plot their next move, Minato ultimately accepts the post and is installed as the Hokage, much to his wife Kushina's joy. As Hokage, he sends Kakashi on a mission to transport a scroll, however, the young jōnin's trauma affects the mission and he is almost killed by his pursuer. Might Guy, however, intervenes and later reports this occurrence to Minato who relates that he was glad he had sent Guy along covertly as well. Meeting Kakashi while he was in the hospital, Minato informs Kakashi that he would like him to enlist in the Anbu under his personal control as his right-hand-man. Heading to the equipment stores for his gear, Kakashi dons his Anbu attire along with his mask, thus beginning his tenure as an Anbu operative.