Two boys meet for the first time as they try to buy a wrecked dinghy from a mysterious old man. He agrees to sell it to them both, and an uneasy alliance is formed, softened when they discover that they have a mutual favourite, the book, Swallows and Amazons. It seems the old man is a fan too.
Upon first inspection of their new prize, the boys discover a simple map, with a single X on it. The family joins in, and they realise that it is the outline of the estuary where they are planning to sail. The X is actually on a tiny island.
This means they are going to have to repair the boat, and learn to sail properly, before they can begin their search. They work together, and along with two older siblings, make their erratic way over to the island. There is a riddle to solve, and it yields up a few coins, and also another clue.
This second pointer is followed, and that too solved, yielding a larger collection of coins, and yet another clue. However, by now they have attracted the attention of a modern-day pirate, who begins to stalk them, in the hope of getting the old man's treasure for himself.
After a few narrow escapes, they eventually they give the pirate the slip, and with their siblings and a borrowed mother, follow another clue, north to the Lake District. They take lodgings in an isolated farmhouse, hidden deep in the interior.
After solving another riddle, the final treasure now seems to be within their grasp, in a secret boathouse. They make the trip over the lake, but the place is full of crates, obviously a store of some sort, but why is it hidden out here? To their horror, the boys discover that the owners of these crates are none other than a gang of criminals, who are seemingly running their very guesthouse. The whole family is barricaded into their rooms while the villains decide how to dispose of them. Only the boys can save them all, but this involves a midnight breakout, and a desperate sail up the lake.