"Mystery of Pain" (ペインの謎, Pein no Nazo) is episode 160 of the Naruto: Shippūden anime.
At Mount Myōboku, Naruto wonders whether the Akatsuki would ever attack Konoha just to find him, while Fukasaku assures him that no such thing would happen; even if it did, Kōsuke, the messenger toad, would alert them. Fukasaku then decides to begin training on the third and final step to using Sage Mode in battle — fusing Fukasaku on his shoulder.
In Konoha, Danzō alerts his Anbu Root unit that they would be hiding underground until Pain's assault comes to an end, adding that they would not be providing any support as the village's crisis would result in his promotion to Hokage, and thus any casualties would be considered as sacrifices for the greater good.
Meanwhile, Inoichi continues his interrogation and probing of Yūdachi's mind, finding a memory in which Yūdachi and Ryūsui delivered a corpse to Konan, who was to deliver it to Pain himself. The Konoha Torture and Interrogation Force is then attacked by Pain's Animal Path, who summons a Giant Rhino to destroy the building. Inoichi recognises her as the corpse in Yūdachi's memories, and flees with Ino and Shizune, who are protected by Katsuyu; Shizune, however, wonders how this Pain is able to summon when Jiraiya had already defeated the one that could. Ibiki and the others decide to attempt defeating the Animal Path, who summons a Giant Ox, a Giant Multi-Headed Dog and a Giant Snake-Tailed Chameleon. The Anbu unit attempts to defeat the dog, only for it to survive and continue its rampage.
Ibiki traps the Giant Rhino using his Summoning: Iron Maiden technique while binding the Animal Path and summoning a torture chamber. He attempts to use pain to extract information, only to realise that the Path's body is cold and responsive as a corpse. Easily breaking free of the shackles, Pain tells Ibiki that he shall know what true pain is, and proceeds to his stomach as a conduit to summon a Giant Drill-Beaked Bird. The Anbu unit attempts to defeat the giant multi-headed dog once more, only for the dog to break free again; the giant rhino also breaks free of the Iron Maiden.