Mystery of Pain

is episode 160 of the Naruto: Shippuden anime.

Synopsis
Konoha continues to fight off Pain while attempting to learn his secret. The Animal Path attacks the ANBU Interrogation Corps, and the Konoha ninja present must fight off a variety of different summons. Meanwhile, Naruto finishes his Sage Mode training and attempts to bond with Fukasaku, so that he can move in Sage Mode while the Toad Sage gathers Nature Energy. However, Fukasaku is rebuffed by the Nine-Tailed Fox, forcing the duo to find an alternate method for Naruto to enter Sage Mode in battle.