Sasori's Real Face

is episode 21 of the Naruto: Shippūden anime.

Synopsis
With his puppet destroyed, Sasori reveals himself to Chiyo and Sakura, looking no older than when he left his village over twenty years ago. Meanwhile, Temari's team, along with Kankurō and Ebizō, continue to make their way towards the Land of Rivers, where Ebizō tells the others of the strongest ninja in Sunagakure's history, the Third Kazekage, and his mysterious disappearance. Back in the Akatsuki lair, Sasori summons the Third Kazekage puppet, whom he had kidnapped and turned into a Human Puppet many years ago. Using its weapons, which are even more dangerous than Hiruko's, Sasori is able to catch Sakura in a cloud of poison gas.

After fighting for about seven episodes, Sakura and Chiyo finally trap Sasori in his final body, to find out from Sasori that the only reason his face hasn't changed is that it he himself is a human puppet. They then find out that he made himself one, and the only part of him that is still human, is his source of chakra, his heart.