When eight-year-old Kirstie brings home a strange egg from the Scottish shore, her family never expects it to hatch. But the next day, Kirstie discovers a baby sea monster swimming in her bathtub! Part horse, part toad, part turtle, and part crocodile, he's the oddest-looking creature Kirstie's family has ever seen. Only Kirstie's grandfather knows what the beastie really is -- a Water Horse!

This lovable creature is the perfect pet. The only trouble is, he can't stop growing! He gets bigger. And BIGGER. And BIGGER. Before long, he's outgrown nearly every loch in sight. Will they ever find a home that's large enough for their Water Horse?