Tintin purchases a model ship at an antiques market. Just after he purchases it two strangers arrive who want to buy the ship. Tintin won't sell it to them even though they offer him ten times what he paid for it. It is a gift for his friend Captain Haddock. The captain is amazed to get the model ship. He shows Tintin a painting of his ancestor, a captain. The captain's ship is visible in the background, and is identical to the model Tintin purchased. The secret to buried treasure is somehow hidden in the model ship, but other parties are also after it...
This particular Tintin book was my favorite when I was a child, mostly because of Captain Haddock. The Captain is continually hollering fake profanities, such as, "Billions of blue blistering barnacles!" I guess that could be a little disturbing now, since the captain acts funny because he is a raging alcoholic (trying to quit though which is a plot point, and I don't think that that is a reason to keep the book from children). This story cuts back and forth in time as bits and pieces of Captain Haddock's family history are shown and trigger new events in the search for treasure.
If you are reading this to help learn German, Tintin comics are good for reading at a German 2 level. There are a lot of words that aren't basic vocabulary but it is still easy to follow the story because the writing and pictures tend to reinforce each other.