Opal Moonbaby is spending a year on Earth. A whole year! Martha can't wait to take her to school, to introduce her to her friends and to recreate all the fun they had during the summer. But things don't turn out quite as expected and 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, enemies from their home planet of Carnelia, are on their way to Earth, planning mischief, Martha begins to worry. But Opal is far too busy making friends to be bothering about those stupid Mercurials. Besides, her eyes would z-ray them immediately and she'd dazzle-kick them all the way back to Carnelia. Wouldn't she?