The Five have heard the stories about treasure on Demon's Rocks, but they never thought they were really true! When they discover a very old, valuable gold coin, the myth becomes real. Who does the treasure belong to? The gang are determined to find out...but so is someone else.
In the Five's nineteenth adventure, Uncle Quentin has an important visitor coming to stay at Kirrin Cottage, a brilliant professor involved in great work. The problem is, Professor Hayling is arriving a week earlier than expected—and with his son Tinker! This is a problem for Aunt Fanny, because George, Anne, Julian and Dick are due to arrive on the same day, and there's just not enough room! To make matters worse, it seems Tinker is a pest of a boy, always zooming about the house pretending he's a car. And he has a monkey called Mischief. With five children and two animals in the house, it's inevitable that there's going to be a commotion of some kind every five minutes, and Quentin and the professor just CANNOT STAND THE NOISE!