The Keroro Platoon members live their lazy days, unconcerned about the invasion. But one day, they are faced with a series of mysterious phenomena breaking out all over the country. Feeling a sense of crisis, the Keroro Platoon put their pride as invaders on the line and take action!. What is the meaning of the mysterious letters they encounter everywhere they go? The largest battle in the history of "Sgt. Frog" begins in the Reiwa era!.