Set over an idyllic English summer holiday, the four Walker children and their sailing rivals, the Blacketts, run amok in their boats against the stunning backdrop of the Lake District.

The Walkers sail the agile Swallow and the Blacketts commandeer The Amazon, making camp on a nearby island.Their world is one of imagination - filled with pirates and canons, where boats are captured and the enemy has to walk the plank.

But when family friend, Uncle Jim is revealed to be a spy, our motley crew are forced to put down their imaginary swords and band together to protect him from his Soviet enemies.