Episode 321
Keroro decides older things are better, and turns everyone's stuff into old-fashioned versions! (Dororo sobs because he's unaffected.) Later on, Natsumi is being suspiciously nice to Keroro. What possible reason could there be?! Source: crunchyroll
Aired 03/07/2010EP 321 - Episode 321
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)