In the midst of World War II, a lonely young boy named Angus MacMorrow lives with his mother and sister on the shores of Loch Ness in Scotland. His father is away fighting in the war, leaving Angus to explore the vast and mysterious surroundings of his home. One day, while wandering along the lake, Angus discovers a strange, ornate egg hidden in the rocks. Curiosity piqued, he takes the egg home, where it soon hatches into a mythical creature that the local legends refer to as a "water horse." Angus names the creature Crusoe and forms an immediate and unbreakable bond with it.

As Crusoe begins to grow at an astonishing rate, it becomes increasingly difficult for Angus to keep his magical friend a secret. Complications arise when the British Army arrives to establish a base at Loch Ness, bringing with them a multitude of dangers not only for the local residents but for Crusoe as well. The presence of the military, along with their equipment and paranoia about enemy forces, puts Crusoe at great risk of being discovered. Meanwhile, Angus must navigate the challenges of maintaining the secret of Crusoe's existence, all while developing friendships, learning the values of trust and loyalty, and coming to terms with the realities of war and loss that surround him.