Julian y Anne llegan a la isla de Kirrin para pasar unos días con su prima George,el padre de esta,el profesor Quentin,dueño de la isla,ha instalado una misteriosa torre donde se dedica a hacer experimentos destinados a convertir la fuerza del mar en energía.

Dos individuos llamados Curton y Jhonson,interesados por las investigaciones del profesor Quentin,deciden robarle los planos y las fórmulas,¿lo coseguiran?.

There's a thief at Kirrin Cottage! The Famous Five think they know who it is, but they need to prove it! Where can they find evidence? The discovery of an old map and very unusual hiding place is all they need to get to the bottom of this mystery and uncover the true culprit!

Los Cinco son invitados a pasar unos días en casa de la familia Lenoir,casa prevista de una torre a la que llaman torre de los contrabandistas.George ha llevado a Timmy,pero al señor Lenoir no le gustan los perros y tienen que esconderlo en uno de los muchos pasadizos secretos que hay en el edificio.

Los Cinco,que desconfían del señor Barbú sospechan que el criado de los Lenoir está relacionado,de alguna manera,con dicho personaje.

What is Uncle Quentin up to, all alone on Kirrin Island? He won't let anyone visit, not even George and the rest of the Famous Five. But Uncle Quentin isn't really alone on the island - someone is watching his every move.

Una gran acampada se prepara para los Cinco.La alegría se desvanece un poco cuando se enteran de que el viejo Luffy,empedernido cazador de insectos,les acompaña.

Aprovechando un descuido del guardabarrera,los Cinco se introducen en un tunel de donde proceden los trenes y aquí comienza su verdadera aventura.

Dick is puzzled when he's woken by a light flashing through his window. Is someone trying to send him a coded message? And when the Famous Five hear of an escaped convict in the area, they are on red alert. The police won't help, so the Five have no choice but to solve the mystery alone.

Camping again! This time the Five have pitched their tent near an old ruined cottage, which looks as though it's been abandoned for years. When Anne hears strange noises at night the others don't believe her ... until they see ghostly lights. Could the cottage be haunted?

Hurrah! It's holiday time, and the Famous Five are spending it at Billycock Hill. Most exciting of all, they've made a new friend - a real pilot! But when he disappears with top secret equipment, the Five are puzzled. Could their new friend be a spy?

There is a ruined castle on Finniston Farm, but only the dungeons remain and no one knows where they are! The Famous Five are determined to find them, and whatever else is hidden inside, but someone else wants to find out too. Can the Five get there first?