One day, while collecting seashells, he discovers what appears to be a large mysterious egg in the sand, which he leaves it in his father's shed. When he returns later, an unknown creature hatches (which he calls 'Crusoe' after Robinson Crusoe) that becomes the fabled Loch Ness Monster. Angus keeps the creature a secret, but eventually tells his sister and (reluctantly) Lewis about it. Lewis explains to Angus that it is a genderless "Water Horse" that lays one egg, then dies before it hatches.
The next day troops of the 12th Medium Regiment Royal Artillery arrive at the house, commanded by Captain Thomas Hamilton — a friend of Lord Killin, who is serving with the Royal Air Force. An artillery battery is set up near the lake as defence against possible attacking or hiding German U-boats and the troops set up camp on the grounds of the house. An anti-submarine net is also raised at the mouth of the lake to prevent the possible entrance of German U-boats into the lake.