Gintama Episode 116: The Older, the Wiser
Episode 116 of 201 of Gintama, aired July 23, 2008, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Kamenashi (the turtle) tells the group about Otohime, the owner of the Dragon Palace, and that they are the only ones who are capable of stopping her plan.