Plot
When siblings Julian, Dick and Anne cannot go for their usual summer holiday to Polseath, they are invited to spend the summer with their Aunt Fanny and Uncle Quentin at their home Kirrin Cottage, in the coastal village of Kirrin. They also meet their cousin Georgina, a surly, difficult girl, who tries hard to live like a boy and only answers to the name George. Despite an uncomfortable start, the cousins become firm friends and George introduces them to her beloved dog Timothy (Timmy), who secretly lives with the fisher boy, James, in the village as George's parents will not allow her to keep Timmy.
On their way to Kirrin Island, George shows her cousins a shipwreck, explaining it was her great-great-great-grandfather's ship. He had been transporting gold when the ship was wrecked in a storm, but despite divers investigating the wreck, the gold was never found. After visiting the wreck, the five arrive on the Island and are exploring the ruined castle when a huge storm blows up, making it too dangerous for them to return to the mainland. While they take shelter on the island, the sea throws up the old shipwreck, grounding it on the rocks surrounding the island. Excited by these developments, they decide to come back at dawn the next day to investigate the wreck before it is discovered.
The following day, the five visit the wreck and discover the captain's cabin, where they find some objects belonging to George's great-great-great-grandfather, including an old box which they take back to Kirrin Cottage. The box proves difficult to open, so they throw it from the highest window of the house. The box breaks open, but the noise disturbs Uncle Quentin who confiscates the box. Not willing to give up their quest, Julian sneaks into Uncle Quentin's study and takes the box, which contains an old map of Kirrin Castle. Later in the further investigation of the map the children find a word INGOTS, which they later understand represents the place in which the lost gold was buried. The children realise it is a treasure map showing the location of the lost gold. After making a tracing of the map and returning the box, they decide to find the gold themselves.
To the children's shock, the box containing the map is sold to an antique collector. The same man also makes an offer to buy Kirrin Island. The children realize he has unearthed the secret map and wants the gold for himself, and so begins a race for the five to get to the gold first. Thinking the children want to spend time at the island before it is sold, Uncle Quentin and Aunt Fanny allow them to go camping there for one week.
Arriving on the island, the five start searching the castle ruins to locate the old dungeons where the gold is hidden. Chasing a rabbit, Timmy falls down an old well, from where the children find the dungeon entrance. Exploring underground, they find the gold in a locked vault however trouble soon arrives, as bad men come to the island hoping to steal the gold. They capture George and Julian, lock them in the dungeons and ask them to write a note to Anne and Dick up on the surface, a note that will lure them back down into the dungeons. George writes the letter and Dick finds something fishy about it, as George has signed her name "Georgina", so Dick and Anne decide not to go down. Unable to find them, the men leave the island, taking the oars from the children's boat so they cannot escape. Dick and Anne use the well shaft to rescue Julian and George from the dungeon, and the children hatch a plan to trap the men when they come back to the island with a boat to steal the gold.
Although the plan goes wrong, the children manage to leave the men stranded on Kirrin Island. They return to the mainland to tell Uncle Quentin, Aunt Fanny and the police what has happened. The gold is recovered, and it is determined that it legally belongs to George's family, making them rich and enabling them to afford everything they have ever wanted. George's only wish is to be allowed to keep Timmy, and her parents agree. George also agrees to go to boarding school with Anne, because she and her cousins have become very good friends and also the school allowed pets.
