Convinced that his father, who had disappeared at sea two years earlier, is still alive, the 12-year-old Felix sets out to find him in the company of Old Tom, a retired fisherman, Squawk, the one-legged parrot and Rover, a cat who acts like a dog. Their journey takes them to the Darkshadow Island, to an underground city where a secret society is hiding, led by the megalomaniac Morgäa, possessor of a priceless treasure.