The professor is accompanied by his son, Tinker, who often has a sudden urge to start imitating vehicular noises and has brought his pet monkey, Mischief.

The ensuing crowded and noisy household upsets the two scientists, prompting Tinker to propose that the children spend their holiday at his abandoned lighthouse at Demon's Rocks, located 10 miles away.

After settling in at the lighthouse, the children meet an elderly retired sailor, Jeremiah Boogle, who tells them of his youthful encounters with three villains who lured ships to Demon's Rocks and plundered the wrecks.

Unable to reach the mainland because of the rising tide, they return to the lighthouse, light its lamp and ring an old warning bell amid a fierce gale to alert the villagers to their fate.

Julian and Dick declare they will recover the treasure for the police and then the children will return to Kirrin Cottage.