Tenchi Bridge Reconnaissance Mission

The Sasuke and Sai arc is from Part II of the series, and spans from chapters 282 to 310 of the manga and episodes 33 to 53 of the Naruto: Shippuden anime. In the third databook, this arc is called the. The anime DVDs for this arc call it the "Long-Awaited Reunion". This arc details Team Kakashi new members Yamato and Sai, and their reunion with the defector Sasuke Uchiha.

Summary
Upon arriving at Konoha, Kakashi was confined to the hospital, making him unable to lead the team for a while. When reporting to Tsunade, they informed her about the meeting with Sasori's spy in ten days. In order to be able to go, Naruto needed to find a replacement for Sasuke. While searching, Naruto was attacked by this "third man" before actually learning who he was. The ninja, later introduced as Sai, had attacked to test Naruto's powers, since he had been sent by Danzo to replace Sasuke. Sai is a member of the now-defunct independent subdivision of ANBU called the Foundation. It is also discovered that some of the village elders were worried about the possibility of Naruto being captured by the Akatsuki. Tsunade convinced them to let him go by assigning Tenzo, an ANBU captain, as the temporary leader of Team Kakashi, under the codename Yamato. They were to travel to the Heaven and Earth bridge in Kusagakure to meet Sasori's spy.

Encounter with Orochimaru
Team Kakashi took on their new mission, and a great amount of tension began to build in the team, especially between Naruto and Sai. Naruto disliked Sai since their interactions were similar to those between himself and Sasuke in the beginning of the series. Making matters worse, Sai nonchalantly mentioned Sasuke's treachery, greatly irritating both of his teammates to the point that Sakura punched him in the face.

Yamato, disguised as Sasori, went to meet the spy. The spy was revealed to be Kabuto Yakushi, but the jutsu binding him to Sasori had been dispelled long ago. Orochimaru appeared, and Kabuto attacked Yamato, destroying the disguise. Orochimaru revealed that he had performed experiments on Yamato and 59 other children, giving them the First Hokage's DNA in an attempt to replicate his Mokuton kekkei genkai. Yamato was the only survivor. Orochimaru then called out the rest of Yamato's team, and taunted Naruto by speculating whom was now stronger: Naruto or Sasuke. Naruto, provoked by the mere mention of Sasuke's name, transformed first into his three-tailed fox state, then into four-tailed state shortly thereafter.

It is revealed that, during the timeskip, Jiraiya was nearly killed by Naruto's four-tailed transformation. The Nine-Tails' chakra becomes so intense that it simultaneously burns Naruto's skin and rapidly heals the burnt tissue. This speeds up cell division dramatically, decreasing Naruto's overall life span.

In his four-tailed state, Naruto fought Orochimaru to a draw. In the end, Orochimaru was only able to throw Naruto back to the bridge. Naruto was unable to distinguish friend from foe, and attacked Sakura when she tried to calm him down. Yamato finally got close enough to use Hokage-style Sixty-year-old Technique - Kakuan Entering Society with Bliss-bringing Hands, a demon-controlling technique, to revert Naruto back to human form.

With Naruto's transformation reversed, Sakura used her medical skills to heal Naruto's severe injuries, still suffering herself from her previous injury. Although Sakura was upset at how little she could do, Yamato stated that it was the strength of her feelings for Naruto that mattered most.

Meanwhile, Sai revealed to Orochimaru that he was a messenger from Danzo. Sai, Orochimaru and Kabuto left to a secret hideout. Yamato watched the encounter with a wood clone, and pursued Sai. Yamato was not fooled when he found a decoy disguised as Sai's corpse.

Naruto woke up to find Sakura injured and the surrounding landscape destroyed. Yamato stepped in to inform Naruto about the real cause of the destruction when Sakura hesitated to tell Naruto the real cause of her injury. In addition, Yamato told Naruto that the only way to save Sasuke and protect the village was to rely on his own chakra, not that of the Nine-Tails.

Meanwhile, Sakura went through Sai's belongings and found his art book. The sketchbook contained drawings of Sai and what the team assumed was his older brother growing up and going through a gauntlet of enemies. On each subsequent page, Sai took the weapons and armor of his previous opponent, as did his presumed brother from the opposite side of the book. The center pages, where the two brothers were supposed to meet, were unfinished.

At Orochimaru's hideout, Yamato revealed that he was able to track Sai through a special plant seed he had hidden in his meal the night before, which responded to an increase in chakra, leading Yamato to him. He gave Sakura and Naruto similar seeds to make sure he could find them if they were separated while infiltrating Orochimaru's base. Once inside, Yamato led the team to a room where Sai was being kept hostage. Sai tried to lie about his mission, but an accidental smile allowed Naruto to call his bluff. Sai then stated that his mission was a failure, and confirmed that Danzo was indeed planning another strike at Konoha. However, he was also acting as a spy for Danzo, since Danzo knew that as soon as Konoha was destroyed, Orochimaru would turn on him. Team Kakashi captured him, and bound him outside of the base with Yamato's Mokuton jutsu. Sai questioned why Naruto was willing to go so far to rescue Sasuke, saying that Sasuke didn't see Naruto as a brother and that Orochimaru was too powerful for them. Naruto replied that he and Sasuke had a special bond, and that he would fight Orochimaru with everything he had to get Sasuke back, even if it meant sacrificing his own life.

At this point, Kabuto showed up and freed Sai, but Sai shocked everyone by helping Yamato immobilize Kabuto. Kabuto was forthcoming, providing information on the interior of the lair, but not Sasuke's actual location. Team Kakashi split up to search for Sasuke, with Naruto teaming with Sai and Sakura with Yamato. Naruto, still tired from his injuries, collapsed, and Sai suggested taking a rest; Naruto was notably less hostile towards Sai at this point. Sai began to talk about his brother, who had resembled Naruto. The short chat soon ended as Orochimaru appeared. Naruto told Sai to find Sasuke while he held off Orochimaru. As Yamato and Sakura arrived to back Naruto up, Orochimaru again mentioned that he did not wish to kill them, since both parties opposed Akatsuki. Orochimaru then left in order to locate Sai. Yamato found a "Bingo Book" (a list of people who were marked for assassination) in Sai's bag, with Sasuke's name in it. Naruto and Sakura realized that another part of Sai's mission was to kill Sasuke, who had been listed as an enemy of Konoha by Danzo. Naruto was speechless, realizing that Sai had duped him with his "fake smiling" act.

Sai managed to find Sasuke in his room, and woke him up. Sasuke, angry at Sai's mention of "bonds", blew up the room. Sakura spotted Sai in the hallway and ran to him. Although she meant to attack Sai, Sasuke quietly said Sakura's name, stopping her in her tracks. Naruto, in his weakened state, finally caught up, coming face to face with Sasuke again.

Meeting Sasuke Again
As Sasuke observed the new Team Kakashi, Sai explained to Naruto and Sakura that his desire to protect the bond between Naruto and Sasuke had overridden the mission Danzo had given him. Sasuke replied that he had broken his bond of friendship with Naruto, and that the only bond he had left was a bond of hatred with his brother.

As both Sai and Yamato moved to try and protect Naruto, Sasuke used Chidori Current, which projected electricity around his body and his new Kusanagi, to repel them. He managed to stun Naruto and Sai, then wounded Yamato. As Sasuke fought Yamato, Naruto underwent a partial initial jinchūriki transformation. In Naruto's mind, the imprisoned Nine-Tails implored him to release the seal. When Naruto refused, the demon scoffed, saying that Naruto had always come to it in times of need, and that Naruto was worthless without its power. Suddenly, Sasuke appeared in front of the Nine-Tails, stunning both of them. Sasuke marvelled at the source of Naruto's power. As Sasuke began to push the Nine-Tails back into its cage, the Nine-Tails then remembered its past history with the Uchiha clan. It stated that Sasuke possesses chakra even more evil than its own, and he is just like Madara Uchiha. The Nine-Tails also warned Sasuke not to kill Naruto, stating that he would regret it.

Outside of Naruto's mind, Yamato pushed the sword out of his wound, and tried to trap Sasuke in a wooden prison. Sasuke blew a hole through the roof of the prison to escape. Naruto yelled out to Sasuke that Orochimaru just wanted to use Sasuke as the next container for his soul, but Sasuke replied that he didn't care as long as it helped to kill Itachi. Yamato then told Naruto and Sakura that he didn't want to hurt Sasuke in front of them, but that he had no choice and would finish the confrontation. Sasuke planned to do the same, but Orochimaru returned just in time to stop Sasuke from performing Kirin. Kabuto told Sasuke to leave the Konoha shinobi alive so that they might continue to oppose Akatsuki. Sasuke told Kabuto that that was a poor excuse to not kill Naruto, but left with them anyway. Naruto was upset by the meeting and by how weak he was in comparison to Sasuke, but Sakura and Sai both assured him that they would help bring Sasuke back.

The team reported back to Konoha, during which Naruto told Tsunade that they had not given up and that they would continue. Pleased by this response, Tsunade assigned the team a new mission. Sai reported to Danzo, who was displeased that Sai had failed his mission. Sai asked to remain with Team Kakashi, and Danzo warned Sai that developing emotions would only lead to hatred and conflicts. Sai had flashbacks of recent events, pondered his own feelings, and realized that he now had friends. He headed off with Naruto, Sakura, and Yamato to begin their new mission.