When siblings Julian (aged 12) Dick (aged 11) and Anne (aged 10) learn that they cannot go on their usual summer holiday trip to Polseath they are invited to their cousin Georgina's house to stay with her mother (Aunt Fanny) and father (Uncle Quentin) who is a clever scientist. We learn Georgina (aged 11) is a very difficult and bossy child. She desperately wants to be a boy. She dresses like one, behaves like one and wears her hair short and curly like one. She answers to George (short for Georgina) and will not answer to her real name because it is a girl's name.
When George's cousins come to stay she introduces them to her beloved dog Timmy, whom she has had since he was a puppy. George tells her cousins how the big island on the beach belongs to her. It is called Kirrin Island and was passed down through her family. On their way to the island they see a shipwreck under the water and George explains how her great-great-great grandfather was travelling on it along with lots of other men. He was transporting gold from one country to another. The boat was shipwrecked during a storm and George tells her cousins how divers have been down into the boat to try and find the gold but they haven't. All four children simply agree the gold floated away in the waters.
They arrive at the island, along with Timmy, and when Timmy chases a rabbit into a well, the children are surprised to see Timmy standing on a stone slab sticking out from the inside of the well. George climbs down into the well to rescue Timmy, just as a horrible thunderstorm begins. When the storm is over the five explore to check the damage and realise the force of the storm has brought the old shipwreck up to the surface of the water. They decide to explore it before anyone else can, so that if they find any gold it will be theirs and no one else's. So they explore the boat and find a small black box looking a bit like a jewellery box, but they cannot get it open.
They take it back to Kirrin Cottage (George's home) and decide a good way to get it open would be to drop it from the highest window of the house. This works and the box cracks open but just as they are about to pick the box up of the floor, Uncle Quentin confiscates it, saying the noise they made opening it was disturbing him from doing his work. However Uncle Quentin only leaves the open box on a shelf on his study so when he falls asleep, Julian sneaks into the study and takes the box. Inside the box they find a map and decide to trace it so they can still have a copy, but return the box to Uncle Quentin. He does this and when they look at the map they notice a Latin word on it. Ingots, meaning gold, and a red cross, meaning that is were the treasure is. They realise the map is a map of Kirrin Island, and the old dungeons on the island. Uncle Quentin announces he has then sold the box to a man, who is thinking about buying Kirrin Island. George is furious and so the four children decide to spend as much time on Kirrin Island as they can, because they realise it will soon be sold. So they decide to camp on Kirrin Island for a few nights, and so take the map with them. Planning on discovering gold, they head down into the dungeons, but they are not alone. Read the book to find out what happens! :)