The leaders of Keron-sei decide that the Keroro Platoon has taken too much time to conquer one backwater planet. They recall Keroro and commence a full-scale invasion. Earth's defenses and civilization collapse completely within 30 hours before the might of the Keron Space Armada.
The Keroro Platoon members are branded as traitors to the cause. Dororo goes into hiding with Koyuki. Tamama is captured, brainwashed and sent to kill Keroro - he fails and is killed by Keroro, his dying wish to see the planet he called home one last time. Kururu sacrifices himself in exchange for Saburo's life. Giroro dies while defending Natsumi and Aki from angry mobs who think they're Keron sympathizers.
With Dororo's help, Keroro escapes from his prison within the Keron mothership and appears before Fuyuki to say goodbye. He then returns to the flagship and activates his Kero Ball's self-destruct mechanism, initiating a chain reaction with the mothership's anti-matter core and utterly wiping out the invading forces.
11 years later, Earth has found peace again. Dororo dies with a smile on his face, basking in the idyllic happiness he had cherished and longed for.
Fuyuki is a mangaka, and the manga he draws is titled Keroro Gunsou.