Present day Scotland. A tale is told to a couple of American tourists by a kindly gentleman in a pub. Angus is a lonely boy who pines for his father, who is serving in the Royal Navy during World War II. Angus, along with his sister and mother, live on an estate that has been billeted by soldiers in the Scottish Highlands, near Loch Ness. The troop's commander has an eye for mom, is suspicious about a mysterious handyman, Lewis, who is also a war hero, and an absurd contention that the Highlands are the real frontline in the war against Germany. Agnus discovers a mysterious egg that hatches a sea creature of Celtic mythology. The creature befriends Angus and will eventually be known as the Loch Ness Monster. Trying to hide the dinosaur-like creature, nicknamed Crusoe, Angus enlists Lewis to transfer it to the lake, where boy and serpent have extraordinary adventures together until human stupidity threatens Crusoe's existence.