Philip and Dinah Mannering and their new friends Jack and Lucy-Ann Trent—along with Jack's irrepressible talking parrot, Kiki—are spending their summer holiday at Craggy-Tops, an ancient, half-ruined house by the sea. Before long, they are drawn to the mysterious Isle of Gloom in the distance and are determined to find a way to get there and explore it. But gradually it becomes apparent that the island may not be as deserted as it's supposed to be.

Who is signalling from the island to the mainland at night? What is going on in the abandoned copper mines beneath the island? And what does the friendly yet enigmatic Bill Smugs know about it all? The four children find themselves caught up in an adventure more thrilling—and dangerous—than they could have imagined!