Sgt. Frog Episode 173: Keroro, Yamato and Kappa
Episode 173 of 358 of Sgt. Frog, aired August 11, 2007, 25 minutes. Available with English subtitles and dub where released.
Episode summary
When Fuyuki goes looking for a kappa in a small town on the outskirts of Tokyo, Keroro ends up being abducted by a boy who takes the sergeant to be his lost (kappa?) friend, Capu. Source: crunchyroll