Aa oneesama wa dare no mono?

When Heaven's main computer, Yggdrassil, crashes due to an infestation of 'bugs', a lone Goddess determines that she needs the help of her big sister...Belldandy! However, that help would force the breaking of the holy contract between Belldandy and Keiichi, and Belldandy would return to Heaven...forever! Urd, however, suspects that her youngest sister may actually have another agenda in mind.