Sgt. Frog Episode 191: Keroro, His Lullaby Legend / Keroro, Decisive Plastic Model Battle!
Episode 191 of 358 of Sgt. Frog, aired December 15, 2007, 25 minutes. Available with English subtitles and dub where released.
Episode summary
When Mom is ready to retire their old vacuum cleaner for a new model, vacuum jockey Keroro soups up the machine so the Hinatas will keep it, but maybe Keroro makes it a bit too powerful... Later, Keroro has forgotten the important thing about building plastic models until a mysterious masked figure challenges him to a model-building competition. Source: crunchyroll