The story begins with a mysterious egg washed up on a Scottish beach, the morning after a great storm. Kirstie and her brother Angus find the egg and take it home. The next day it has hatched into a tiny greeny-grey creature with a horse´s head, warty skin, four flippers and a crocodile´s tail. The baby sea monster soon becomes the family pet - but the trouble is, it just doesn´t stop growing!