Chibi Kero: Kerobouru no Himitsu!?

As a child, Keroro wants nothing more than to impress Pururu. When Giroro gets hold of his brother Garuru's Kero Ball, Keroro eagerly shows off the mysterious device, only to press the wrong controls and trap himself, Giroro, Pururu, and another friend inside it.

The four tadpoles must clear a bizarre pinball-like maze to escape, while outside a young Kururu discovers the ball and hears their pleas. He agrees to help in exchange for curry, but one careless use of the device turns that simple payment into the strange accident that gives Kururu his familiar yellow appearance.