Mizuki Tracking Mission

Spanning episodes 142 through 147, this filler arc sits just after Part I's Land of Rice Fields Investigation Mission, ahead of the Bikochu Search Mission. It brings back Mizuki, the Academy instructor who betrayed Konoha in the series' opening, now returned as a far deadlier threat.

Primed to begin three years of training under Jiraiya, Naruto is livid to learn Tsunade has instead sent the Sannin off to gather intelligence. After Jiraiya slips away, Naruto eavesdrops on one of Tsunade's meetings and discovers that Mizuki has escaped custody and is tied to Orochimaru. Seeing yet another possible path to Sasuke, Naruto sets off after the fugitive and links up with Iruka. When they find Mizuki, his newfound strength is alarming, and Iruka directs Naruto to take on the Legendary Stupid Brothers while he himself faces Mizuki. Crushed by the brothers' raw power, Naruto is rescued by Team 10, who take over the fight so he can reach Iruka.

Along the way Naruto meets Tsubaki, Mizuki's former lover, and with her help the pair locate Mizuki's hideout, where they find he was supplied instructions for a performance-enhancing drug. As Team 10 nears defeat against the overwhelming brothers, Tsunade arrives and convinces the pair that Mizuki had deceived them, sending them back toward the prison. Mizuki then swallows the drug and swells into a monstrous version of himself, overpowering both Naruto and Iruka.

Noticing that Mizuki's speed drops sharply as he channels ever more chakra into brute strength, Naruto and Iruka exploit the flaw with a precisely timed maneuver that finishes in a Rasengan. Once Mizuki is subdued and interrogated, it turns out he knew nothing of Orochimaru's whereabouts, leaving Naruto's hopes of a lead on Sasuke unfulfilled.