On the shore of Loch Ness, Scotland, Angus (Alex Etel) finds an unusual egg. When it hatches, it releases an unexpected surprise: a water horse, the legendary creature from Scottish mythology. While the boy tries to keep its existence hidden from his mother (Emily Watson), he and his new pet, Crusoe, quickly become inseparable. But as the water horse grows larger, eventually becoming the fabled Loch Ness monster, Angus must protect his friend from those who would want to do it harm.