Tenchi Bridge Reconnaissance Mission

The Sasuke and Sai arc is from Part II of the series that 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".

Summary
Upon arriving at Konoha, Kakashi is confined to the hospital, making him unable to lead the team for a while. When reporting to Tsunade, they inform her about the meeting with Sasori's spy in 10 days. In order to be able to go, Naruto needs to find a replacement for Sasuke. While searching, Naruto is attacked by this "third man" before actually learning who he is. The ninja, later introduced as Sai had attacked to test Naruto's powers, since 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 are worried about the possibility of Naruto being captured by the Akatsuki. Tsunade convinces them to let him go by assigning Tenzo, an ANBU captain, as the temporary leader of Team Kakashi under the code name Yamato. They are to travel to the Heaven and Earth bridge in Kusagakure to meet Sasori's spy.

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

Yamato, disguised as Sasori, goes to meet the spy. The spy is revealed to be Kabuto Yakushi, but the jutsu binding him to Sasori had been dispelled long ago. Orochimaru appears and Kabuto attacks Yamato, destroying the disguise. Orochimaru reveals 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 calls out the rest of Yamato's team and taunts Naruto by speculating whom is now stronger: Naruto or Sasuke. Naruto, provoked by the mere mention of Sasuke's name, transforms first into his three-tailed fox state, then into four-tailed state shortly thereafter.

It is revealed that during the time skip, Jiraiya was nearly killed by Naruto's four-tailed transformation. The demon fox's 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 fights Orochimaru to a draw. In the end, Orochimaru was only able to throw Naruto back to the bridge. Naruto is unable to distinguish friend from foe and attacks Sakura when she tries to calm him down. Yamato finally gets 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 uses her medical skills to heal Naruto's severe injuries, still suffering herself from her previous injury. Though Sakura is upset at how little she can do, Yamato states that it is the strength of her feelings for Naruto that matters most.

Meanwhile, Sai reveals to Orochimaru that he is a messenger from Danzo. Sai, Orochimaru and Kabuto leave to a secret hideout. Yamato watches the encounter with a wood clone and pursues Sai. Yamato is not fooled when he finds a decoy disguised as Sai's corpse.

Naruto wakes up to find Sakura injured and the surrounding landscape destroyed. Yamato steps in to inform Naruto about the real cause of the destruction when Sakura hesitates to tell Naruto the real cause of her injury. In addition, Yamato tells Naruto that the only way to save Sasuke and protect the village is to rely on his own chakra, not that of the demon fox.

Meanwhile, Sakura goes through Sai's belongings and finds his art book. The sketchbook contains drawings of Sai and what the team assumes is his older brother growing up and going through a gauntlet of enemies. On each subsequent page, Sai takes the weapons and armor of his previous opponent, as does his presumed brother from the opposite side of the book. The center pages, where the two brothers are supposed to meet, are unfinished.

At Orochimaru's hideout, Yamato reveals that he was able to track Sai through a special plant seed he had hidden in his meal the night before that responds to an increase in chakra, leading Yamato to him. He gives Sakura and Naruto similar seeds to make sure he can find them if they're separated while infiltrating Orochimaru's base. Once inside, Yamato leads the team to a room where Sai is being kept hostage. Sai tries to lie about his mission, but an accidental smile allows Naruto to call his bluff. Sai then states that his mission is a failure and confirms that Danzo is indeed planning another strike at Konoha. However, he is also acting as a spy for Danzo, since Danzo knows as soon as Konoha is destroyed, Orochimaru will turn on him. Team Kakashi captures him and binds him outside of the base with Yamato's Mokuton jutsu. Sai question's why Naruto is willing to go so far to rescue Sasuke, saying that Sasuke doesn't see Naruto as a brother and that Orochimaru is too powerful for them. Naruto replies that he and Sasuke have a special bond, and that he will fight Orochimaru with everything he has to get Sasuke back, even if it means sacrificing his own life.

At this point, Kabuto shows up and frees Sai, but Sai shocks everyone by helping Yamato immobilize Kabuto. Kabuto is forthcoming, providing information on the interior of the lair, but not Sasuke's actual location. Team Kakashi splits up to search for Sasuke, with Naruto teaming with Sai and Sakura with Yamato. Naruto, still tired from his injuries, collapses, and Sai suggests taking a rest; Naruto is notably less hostile towards Sai at this point. Sai begins to talk about his brother, who resembles Naruto. The short chat soon ends as Orochimaru appears. Naruto tells Sai to find Sasuke while he holds off Orochimaru. As Yamato and Sakura arrive to back Naruto up, Orochimaru again mentions that he does not wish to kill them since both parties oppose Akatsuki. Orochimaru then leaves in order to locate Sai. Yamato finds a "Bingo Book" (a list of people who are marked for assassination) in Sai's bag, with Sasuke's name in it. Naruto and Sakura realize that another part of Sai's mission is to kill Sasuke, who had been listed as an enemy of Konoha by Danzo. Naruto is speechless, realizing that Sai has duped him with his "fake smiling" act.

Sai manages to find Sasuke in his room, and wakes him up. Sasuke, angry at Sai's mention of "bonds," blows up the room. Sakura spots Sai in the hallway and runs to him. Though she means to attack Sai, Sasuke quietly says Sakura's name, stopping her in her tracks. Naruto, in his weakened state, finally catches up, coming face to face with Sasuke again.

Meeting Sasuke Again
As Sasuke observes the new Team Kakashi, Sai explains to Naruto and Sakura that his desire to protect the bond between Naruto and Sasuke has overridden the mission Danzo gave him. Sasuke replies that he has broken his bond of friendship with Naruto and that the only bond he has left is a bond of hatred.

As both Sai and Yamato move to try and protect Naruto, Sasuke uses Chidori Current, which projects electricity around his body and his new Kusanagi, to repel them. He manages to stun Naruto and Sai, then wound Yamato. As Sasuke fights Yamato, Naruto undergoes a partial demon fox transformation. In Naruto's mind, the imprisoned demon fox implores him to release the seal. When Naruto refuses, the demon scoffs, saying Naruto has always come to him in times of need and that Naruto is worthless without his power. Suddenly, Sasuke appears in front of the demon fox, stunning both of them. Sasuke marvels at the source of Naruto's power. As Sasuke begins to push the demon fox back into its cage, the demon fox then remembers his past history with the Uchiha clan. He states that Sasuke may possess chakra even more evil than his own, "just like that Madara Uchiha." The fox also warns Sasuke not to kill Naruto, stating that he would regret it.

Outside of Naruto's mind, Yamato pushes the sword out of his wound and tries to trap Sasuke in a wooden prison. Sasuke blows a hole through the roof of the prison to escape. Naruto yells out to Sasuke that Orochimaru just wants to use Sasuke as the next container for his soul, but Sasuke doesn't care as long as it helps kill Itachi. Yamato then tells Naruto and Sakura that he didn't want to hurt Sasuke in front of them, but he has no choice and will finish the confrontation. Sasuke plans to do the same, but Orochimaru returns just in time to stop Sasuke from performing Kirin. Kabuto tells Sasuke to leave the Konoha shinobi alive so that they might oppose Akatsuki. Sasuke tells Kabuto that's a poor excuse to not kill Naruto, but leaves with them anyway. Naruto is upset by the meeting and by how weak he is in comparison to Sasuke, but Sakura and Sai both assure him that they will help bring Sasuke back.

The team reports back to Konoha, during which Naruto tells Tsunade that they have not given up and will continue. Pleased by this response, Tsunade assigns the team a new mission. Sai reports to Danzo, who is displeased that Sai has failed his mission. Sai asks to remain with Team Kakashi and Danzo warns Sai that developing emotions will only lead to hatred and conflicts. Sai has flashbacks of recent events, ponders his own feelings, and realizes that he now has friends. He heads off with Naruto, Sakura, and Yamato to begin their new mission.