The Forgotten Island

is episode 227 of the Naruto: Shippūden anime.

Summary
While on their voyage to the Land of Lightning, a giant bird swoops down and kidnaps Guy. The team pursues the bird and are led to a volcanic island. Upon arrival, Aoba spots a mysterious girl watching them and decides to pursue her while Yamato and Naruto search for Guy. During their search, Naruto and Yamato encounter many giant creatures, including the bird that kidnapped Guy. They eventually locate the bird's nest where they find Guy, who was actually kidnapped to babysit the bird's chicks. Meanwhile, as Aoba pursues the girl, he discoverers the animals on the island are actually bred to be used by shinobi as summons, as well as for research dedicated to developing the ultimate summon. He continues to follow her, and she leads him inside the volcano to find a huge monster asleep inside an abandoned research lab. Using a mind reading technique, he discovers the monster was too powerful to be contained and had devoured many of the islands summons, as well as the researchers, before going to sleep.

Meanwhile, the bird takes Yamato, Guy, and Naruto to the volcano where they encounter Aoba, and the beast suddenly awakens, going on a rampage. The monster is powerful and uses genjutsu as well as Lightning Release. Naruto decides to enter Sage Mode while Yamato protects him as Guy and Aoba fight the beast in order to buy some time. Guy manages to knock the beast into the heart of the volcano, but the monster can also fly, and it starts trying to escape. The bird which kidnapped Guy comes down and attacks the monster along with the other summons. The girl, actually the spirit of the girl who the researchers used to restrain the beast until it grew too strong, appears and manages to immobilise the beast, with Naruto finishing it off with Wind Release: Rasenshuriken.

Trivia

 * When Naruto goes into Sage Mode he has elongated canines, but remains with standard Sage Mode eyes.
 * When the summoning animals were trying to prevent the Ultimate Summoning Animal from escaping the volcano, a couple of chameleons bearing a striking resemblance to Pain's Giant Snake-Tailed Chameleon can be seen.