Since the adventures Jim Button and Lucas the engine driver had in the first volume, life on Lummerland carries on as usual. When one day, the postman bumps his boat against New Lummerland because he didn't see it, the decision is taken to build a lighthouse. Unfortunately, however, Lummerland is far too small for such a thing. A lighthouse that is both big and small is what is needed, like the sham giant Mr Tur Tur.
The two explorers set off over the water with Emma and Molly for the "End of the World" dessert to collect Mr Tur Tur and take him back to Lummerland. Unfortunately, when they get to the dangerous magnetic rocks, they have to leave Molly behind. When they return with Mr Tur Tur to where they had left Molly, the train engine has disappeared. They decide to convert Emma into a submarine in order to look for Molly deep down in the sea. After a while, however, the travellers start to get a little drowsy for they are running out of air in the driver's cab. Before they lose consciousness, they see underwater the ruins of a mysterious town. When they come to again, they are on Lummerland. Mr Tur Tur takes up his position as the lighthouse. Then a wrongly addressed letter from the Wild 13 is delivered to Mrs Mahlzahn. The letter tells them that it was the Wild 13 that took Molly from the magnetic rocks. In the meantime, Mrs Mahlzahn has completed her transformation into the Golden Dragon of Wisdom. And so Jim and Lucas travel to Mandala to be at the pagoda when she awakens. The dragon gives them some advice on where to find the Wild 13 and get Molly back and how Jim can solve the mystery of where he comes from. They set out in a sea-blue boat and it's not long before they come across the boat belonging to the Wild 13, which they then secretly follow. But the pirates manage to seize the other ship and Jim is horrified to witness the sinking of his Molly.
In "The Land That Mustn't Be" Jim discovers that in reality he is Prince Myrrh of Jamballa. The pirates of the Wild 13 and their prisoners go to China and the Golden Dragon of Wisdom, who frees them from the delusion that they are the Wild 13, for in actual fact there are only 12 of them. The dragon tells them that the sunken land of Jamballa will emerge again when the former Wild 13 sink the "The Land That Mustn't Be". When they have done that, Jamballa rises again out of the waves in the sea and Lummerland is its highest summit. Jim can take possession of his country as Jamballa and marry Li Si. Molly, too, comes back to the surface – the sea people found her in the deepest part of the ocean.
Jim Button and Lucas the engine driver are best friends and have been ever since the day when little Jim came to the island of Lummerland in a parcel. They are a dauntless team and as such survive all sorts of quaint and exciting adventures together with bogus giants, half-dragons and lots of other unusual creatures. Nothing is too far away for them, no water too deep and no mountain too high when they want to reach their goal. On the occasion of its 55th anniversary, this award-winning classic of children's literature has been given a charming new outfit with Mathias Weber's colourisation.