Opal Moonbaby is an alien and spending a year on Earth. Martha can't wait to take her to school, to introduce her to her friends but before Martha knows it, Opal is off making new friends, doing new things and throwing herself into life on Earth - and Martha can't keep up. When Opal's Uncle Bixie warns them that the nasty Mercurials are on their way to Earth, planning mischief, Martha begins to worry. But Opal is far too busy making friends.