From ANN:

On a walk home, Keroro and Fuyuki stumble upon a hidden shrine. Upon exploring it, they end up causing damage to a shining orb within it. That night, Keroro finds a big X on his face, which causes him considerable distraught. The next morning, a breaking news story shows that the X has spread to more than just Keroro, giving the people of Tokyo telepathic abilities, including the Hinata Household and the Keroro Platoon. The Earthlings seem to think this is a great gift, but the mark may actually be part of a Pekopon invasion older than time itself.

Wikipedia entry