"The Mystery of Tobi" (トビの謎, Tobi no Nazo) is episode 139 of the Naruto: Shippūden anime.
In the woods, the Eight Man Squad find themselves unable to defeat Akatsuki member Tobi. Despite Naruto managing to hit him with a Rasengan, Yamato binding him with his Wood Release and Shino using his Secret Technique: Insect Sphere, all of those attacks manage to go through Tobi. Kakashi is forced to uncover his Sharingan, but does not go head to head with Tobi as Zetsu arrives, announcing to everyone that Sasuke has managed to kill Itachi Uchiha, shocking the group. Tobi immediately drops his high-pitched voice and goofy demeanour and reverts to a cold, harsh tone, telling the group that he will see them later. He and Zetsu then head to the Uchiha Hideout in order to retrieve Sasuke and Itachi's bodies; however, before he leaves, he stares at Kakashi, and Kakashi notices in shock that Tobi possesses the Sharingan as well.
At the hideout ruins, Tobi asks Zetsu if he had recorded the event, as he wishes to analyse it later. Taking the bodies of the Uchihas, they flee to their hideout. Zetsu also stops the fight between Kisame and Suigetsu, notifying them of Sasuke's victory and inviting Hebi to the Akatsuki's hideout in the East. Kisame, however, chooses to resign from the Akatsuki following Itachi's demise. Meanwhile, using Hinata's Byakugan, the Eight Man Squad reach the hideout, only to find themselves blocked by Itachi's Amaterasu; Yamato uses his Earth Release: Earth Flow Divide to provide them a safe passage. As they stand on the ruins of the hideout, they realise that Sasuke is gone, while Naruto cries about how he is never able to catch up to Sasuke.
At their hideout, Tobi reveals to Sasuke that he is a survivor of the Uchiha Clan Downfall and partially removes his mask in order to show Sasuke his Sharingan. However, Sasuke's left Sharingan immediately activates and transforms into Itachi's Mangekyō Sharingan, igniting Tobi with Amaterasu. Fleeing and returning moments later, Tobi explains that Itachi must have passed down the Amaterasu to Sasuke and have it rigged to go off if he made eye contact with his Sharingan. Sasuke wonders why Itachi would have done such a thing, and Tobi replies that he knows many things about Itachi that Sasuke does not, including the fact that Itachi was trying to protect Sasuke.