"Infiltration: The Den of the Snake!" (潜入!毒蛇の巣窟, Sennyū! Dokuhebi no Ajito) is episode 47 of the Naruto: Shippūden anime.
In Orochimaru's hideout, Sai finally comes face-to-face with Sasuke, the man that Naruto and Sakura have been searching for. Sasuke is impatient, as he was waiting for Orochimaru to teach him a new technique. When Sai attempts to introduce himself to Sasuke, Sasuke uses his Sharingan to place him under a unique genjutsu, which fills the usually emotionless Sai with fear and brings him down to his knees. Sai regains his composure and tells Sasuke that he must be the one Naruto is looking for. The mention of Naruto's name piques Sasuke's interest, but the latter merely rebuffs that he sounds familiar. Sai then tells Sasuke that Sakura had told him that Naruto thought of him as a brother; Sasuke responds that he only has one brother, and that is the one he must kill. With that, Sasuke disappears, and Orochimaru heads off as well, leaving Sai with Kabuto. Before he leaves, he gives the contents of Danzō's envelope to Kabuto and tells him to prepare a Bingo Book. Kabuto opens the envelope and to his surprise, discovers that the contents are the personnel files of every Anbu member under the Hokage's command, including those assigned to the Hokage Guard Platoon.
On the outside, Yamato, Naruto and Sakura meet up with Yamato's wood clone, who tells them that Orochimaru's hideout is right behind a rock. Yamato absorbs his wood clone and gives the team its remnants - tracking seeds, which the team swallows. Yamato tells them that he planted one in Sai's food and one on Sai's clothing, which is how the wood clone tracked him. Yamato then uses his Earth Release: Tunnelling Technique to create a large hole in order to sneak into the base undetected. He tells Sakura to follow him first. As Naruto prepares to jump, he muses that he has finally caught up to Sasuke.
The Konohateam reaches a dead end; Naruto suggests destroying the wall with his Rasengan, but Sakura reminds him that in doing so, he would alert Orochimaru to their presence. Yamato uses his Wood Release: Hand Tool Manipulation technique to transform his finger into a drilling tree branch, eventually shaping it into a block of wood. The process opens a hole in the wall for the trio to infiltrate through, and Yamato warns the team to limit their usage of chakra as to avoid detection.
Kabuto leads Sai to his room, telling Sai to be quiet unless needed. Not trusting him as he is a member of Root, Kabuto locks Sai inside. After Sai inspects the room for any sign of surveillance, he takes out two scrolls from his person and summons a Bingo Book. Kabuto remembers his Bingo Book task and realises that he left the envelope in Sai's room, and decides to head back. He begins walking down the corridor, unaware that Team Kakashi is running down an adjacent corridor looking for Sai as well. In his room, Sai begins to peruse the contents of the book and freezes as he hears the sound of his door unlock.