Julian, Dick and Anne are coming to Kirrin to stay for a couple of weeks, but are beaten by Quentin's colleague Professor Hayling and his son Tinker (and monkey Mischief) on their way. There isn't enough room at Kirrin Cottage for everybody to stay, so Tinker suggests he and the Five go to his lighthouse at Demon's Rocks up the coast.
On arrival at the lighthouse, they settle in, and meet the local character Jeremiah Boogle, introduced to the Five by a local taxi driver Jackson, who tells them stories of olden time smuggling which went on in the area. He tells them about the three Wreckers who wrecked ships by luring them towards the Demon's Rocks deliberately and smuggled the wealth out of them. A petty burglary at the lighthouse introduces the two villains - Jacob and Ebenezer - who also show tourists through the old Wreckers' Cave(s). During their own inspection with Jeremiah, Mischief runs off into the tunnels and subsequently appears with a gold coin. Great excitement ensues - he's found a long lost hoard of treasure, but where? Unfortunately Ebenezer (Ebby) is nearby, and also realises that treasure is in the vicinity. The turning tide means that they have to leave the cave, to return later to look for the treasure. Ebby and Jacob ensure that they can't do this by locking them in the lighthouse - but, with the aid of an old map, Julian discovers that it is possible to enter the tunnels through the lighthouse foundations. He and Dick do so, and in the space of a few pages locate the treasure. They then return to the lighthouse and that night light up the old lamp to let people know they are there and need help. Julian and Dick also lug the old warning bell up to the top of the lighthouse and, in a ferocious gale, bang it loudly to reinforce the light's message. Next morning they are rescued, and the tale ends with Julian and Dick promising to fetch the treasure and Anne planning to pack in readiness to go home.