Ag éirí as na heachtraí in An tAonbheannach Mistéireach, tá Tintin agus a chairde ag triall ar an Muir Chairib chun teacht ar chiste caillte Raga Rua. Tagann siad ar an long bháite, An tAonbheannach, ach cá bhfuil an taisce luachmhar? Ní aimsíonn siad ach seanchisteog agus roinnt doiciméad inti. Tá cúpla rún mistéireach sna doiciméid sin, áfach, a choinneoidh gnóthach iad.

Tintin and his friends head for the Caribbean in search of pirate Raga Rua’s lost treasure, and the ship An tAonbheannach. They find the sunken vessel beneath the waves, but the treasure has vanished. All that remains is a chest full of manuscripts — manuscripts which reveal surprising information!