"The Top-Secret Mission Begins!" (極秘任務スタート, Gokuhi Ninmu Sutāto) is episode 41 of the Naruto: Shippūden anime.
Naruto gets more enraged in his three-tailed form, and throws Kabuto away, who inadvertently knocks Sakura out too. Naruto's anger eventually escalates to him destroying the Tenchi Bridge and he takes to a forest to battle Orochimaru. Sai creates an ink bird and flies beneath the collapsing bridge to the site of Naruto and Orochimaru's battle, instead of catching a falling Sakura. Yamato saves her by using his Wood Release, and creates a wood clone to follow Sai. In the forest, Naruto's ever-increasing aggression causes his blood to mix with the Nine-Tails' chakra and transform him into a four-tailed fox.