Sgt. Frog Episode 106: Fuyuki & Natsumi - Convenient Invaders / Giroro - A Time for Hobbies
Episode 106 of 358 of Sgt. Frog, aired April 21, 2006, 25 minutes. Available with English subtitles and dub where released.
Episode summary
When Fuyuki and Natsumi stumble into a secret underground alien city, they go undercover as aliens, but with the frogs there too, chances are good that their cover will be blown! Then, after much prodding by Keroro, Giroro tries to find a hobby that suits him. Source: crunchyroll