From Syuura:

Long ago, Tama-tan met a boy who gave her an object and told her to wait for him. Yet, as seven years passed, Tama-tan still has not seen this boy and begins to doubt his promise. Just when she suspects that day was simply an ordinary day, she is confronted by the shocking appearance of an UFO. Finally she once again sees this bunny-eared boy... only to discover that he is in search of the "Lunar Ladies", which are candidates to becoming the Moon Princess. Unexpectedly, Tama-tan becomes one of them. Yet, what is the true purpose of becoming this moon princess?