Episode 318
Keroro and company do everything they can to get the masked Dororo to show his mouth, a sight they've never seen before. Later, Mois's pen pal from space visits her on Earth and intends to carry out an old promise to help her friend destroy the planet. Source: crunchyroll
Aired 12/06/2010EP 318 - Episode 318
Sgt. Frog
Keroro is a frog-like alien sent from his home planet on a mission to conquer Earth. But when his cover is blown, his battalion abandons him and he ends up in the home of the Hinata family. There, he's forced to do household chores and sleep in a dark basement that was once supposedly a prison cell haunted by the ghost of an innocent girl. He even spends his free time assembling Gundam model kits. During his stay, Keroro meets up with subordinates who were also stranded during their failed invasion.
(Source: Anime News Network)