Sgt. Frog Episode 124: Keroro, That Treasured Summer
Episode 124 of 358 of Sgt. Frog, aired August 25, 2006, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Keroro, Dororo, and Giroro are reminded of the summer vacation day they went to Planet Hiquesviel as kids, to search for Viper pirate treasure. Source: crunchyroll