"Mystery of Pain" (ペインの謎, Pein no Nazo) is episode 160 of the Naruto: Shippūden anime.
Konoha continues to fight off Pain while attempting to learn his secret. The Animal Path attacks the Konoha Torture and Interrogation Force, and the Konoha ninja present must fight off a variety of different summons. Ibiki captures Animal Path in a Summoning: Torture Chamber, and tries to extract information about the enemy but to no success. Instead, torture is turned against him as Animal Path uses Ibiki's stomach as a conduit for summoning the Giant Drill-Beaked Bird. In the end, the Animal Path unleashes all of the summons, which led to more trouble for the Konoha shinobi to handle. Meanwhile, Naruto finishes his Sage Mode training and attempts to bond with Fukasaku, so that he can move in Sage Mode, while the toad gathers Nature Energy. However, Fukasaku is rebuffed by the Nine-Tails, forcing them to find an alternate method for Naruto to enter Sage Mode in battle.