Should I Spa or Should I Go Now?

Keroro receives a letter from his father, rumored by many to be Keron's most fearsome soldier and commander, now planning to visit Earth. Keroro is forced to engineer a massive coverup for his lackluster 'invasion' of Earth, or risk his father's wrath.
Keroro's newest scheme involves creating his own onsen, by digging a new hot spring from the Keron underground base. After drilling underground in the Magma Swimmer drill tank, the Keronians discover buried alien relics left behind by an ancient invasion force, and chaos erupts as the ancient machines reactivate on their own and attack the Keronians - the ensuing battle leaves the Magma Swimmer exhausted of power, and the Keronians without any way to return to the surface...