From Dark Horse:

Keiichi Morisato was a typical college student, a failure with women, struggling to get through his classes, and in general living a pretty nondescript life. That is, until he dialed the wrong number and accidentally summoned the Norse goddess Belldandy. Not believing Belldandy was a goddess and that she could grant his every wish, Keiichi wished for her to stay with him forever. As they say: Be careful what you wish for! Now bound to Earth and at Keiichi's side for life, the lives of this most un-normal goddess and most lucky human will never be the same again.